The idea is to be able to lower one rover that's larger or at least heavier (perhaps better instrumented, or more robustly powered, or equipped with spinning blades to fend off other Battlebots) than Perseverance. If doubling the mass of the skycrane only allows increasing the rover mass by 1.7, that's not a show stopper, and it's certainly no proof that a larger rover than Perseverance is impossible.
Here is the problem with that
Let's suppose that this fantasy of theprestige's of building a twice-as-big rover, and a bigger, more powerful sky-crane, and to then launch them separately into orbit and somehow assemble them in LEO, is all feasible (I think its impossible, at least at our, and any realistically foreseeable, level of technology).
OK, so you have now built your bigger, heavier rover, which required you to build a bigger, heavier and more powerful sky crane, capable of lowering the greater weight. That means your sky crane has more rocket engines to provide the additional required thrust, and more fuel, which means bigger fuel tanks to carry the extra fuel adding more weight. Now your your whole spacecraft (cruise stage + back-shield + sky-crane + rover + heat-shield) is in LEO orbiting at 28,000 kph. This velocity has to be increased using a TMI burn so that it can gets its ass to Mars.
Oh, hang on! That TMI burn now has to provide a bigger kick to the spacecraft's orbital velocity than it would have for the Curiosity or Perseverance setups, because our spacecraft is much heavier. There are only two ways that bigger kick can be achieved -
1. Bigger, more powerful, and therefore heavier rockets to deliver more thrust during the TMI burn... more weight.
2. A longer TMI burn, which will require more fuel, and therefore, higher capacity tanks, which results in.. more weight.
Looks like you will be needing a 3rd launch just to put the boost stage into orbit!
So far, we're not winning - two single sized rovers require two single launches, but now our sooper dooper, double-sized rover needs three launches. This is beginning to look like a bad idea, and it doesn't stop there because we haven't even come up against the real show stopper yet.... Mars atmosphere EDL - Entry, Descent and Landing.
In order to kick our spacecraft out of LEO, we had to boost its velocity. Pretty much any TMI burn to Mars during a Hohmann transfer window is going to result in a Mars atmosphere interface velocity of somewhere between 16 and 28 thousand kph (the slowest on record was Viking at 16K and the highest was Mars Pathfinder at 27K). However, our spacecraft is much heavier than anything we have ever previously sent to land on Mars so its going to take much longer to slow down in the atmosphere, and that means it will get hotter, so it will need a bigger, more robust heat shield (oops) which adds more weight. And then there is the parachute.... it will it have to arrest a heavier load (so it has to be bigger, and so will its housing... yet more weight)
But wait.... there's more
I'm sure there are complexities inherent in a larger skycrane with more (not larger) thrusters and cables/winches, some I probably haven't thought of. Obviously literally attaching two of the old design together without modification, which they were never designed for, wouldn't actually work. But your claim is that no skycrane design could work with a rover payload significantly larger than Perseverance, and your gesturing vaguely toward the square-cube law doesn't seem sufficient evidence for that claim.
This document talks about the plan after Curiosity and before Perseverance to land a Curiosity-like rover in 2018, and while it talks mostly about the seasonal atmospheric pressure issues with regard to landing zone altitude limits and landing ellipse accuracy, it does briefly address the mass issue.
Warning: This is a 3.9MB pdf
https://www.nap.edu/resource/13117/App G 10_Mars-Sky-Crane.pdf
2.4 Mass Delivery Capabilities of the MSL-Derived Sky Crane
The delivery capabilities of an
MSL-derived EDL/Sky Crane system*1 for the 2018 opportunity have been estimated by applying the current understanding of the system’s sensitivities and adjusting for conditions expected in the 2018 arrival season. Additionally, the performance impact of several options for improving landed precision has been estimated based on landing precision studies commissioned by the Mars Program in fiscal year (FY) 2009. The options and their impact on performance are described below.
The 2018 opportunity would afford a more favorable part of the seasonal pressure cycle for landed mass than MSL’s 2011 opportunity; however, the increased risk of exposure to dust events would partially offset the increased mass capability and also negatively impact landing precision. As shown in Table 2-2, after accounting for the positive and negative atmosphere impacts,
the delivery mass capability is estimated to be 1,050 kg, roughly 100 kg more than MSL.*2 Dust impact on wind fields would cause additional variability in the amount and direction of wind drift experienced while on parachute. This would result in an increase to the landing ellipse diameter to 30 km versus MSL’s 25 km ellipse.
NOTE: MOLA - Mars Orbiter Laser Altimeter, a sort of sea-level reference for Mars
Several options for improving landing precision have been considered. Improving navigated state knowledge at entry, through improved attitude initialization knowledge and through either spacecraft to spacecraft navigation or optical navigation, would improve expected landing precision. The improved state knowledge would enable guided entry to be more accurate in controlling range to target; this would result in an estimated 22 km landing ellipse.
Adding a range-based parachute deployment trigger, rather than a simple velocity trigger, to the enhanced state knowledge approach could further increase landing precision. Often dubbed “smart chute,” this option would deploy the supersonic parachute when the desired range flown has been met,
resulting in significant precision improvement.
Unfortunately, this option would come at the expense of landing elevation capability, because parachute deployment must be delayed from the maximum allowable inflation Mach to allow flexibility to deploy using range information.*3 The landing elevation expense would depend on the atmospheric conditions and correlations for a given landing site, but could be estimated at approximately 2 km for most cases. The improved ellipse for the 2018 opportunity is estimated at 15 km in diameter
*1 "MSL-derived EDL/Sky Crane system" - any system using a parachute, retro-rockets and a tether to lower the payload.
*2The mass capability of the the above system and
any system derived from it is about 1050 kg.
*3the lack of atmospheric density means that even the target landing zone altitude is critically affected by how long the spacecraft takes to slow down in the atmosphere. Too long, and the skycrane is too low to for the ranging system to find the landing zone and navigate to it.
We have now reached the limits of what is possible. This is not the limits of those two sky-cranes, it is the limits of this system as a whole - about 1050 kg is the most you can land on Mars using the system as described. Both Curiosity (899kg) and Perseverance (1,025 kg) used precision steering, a huge supersonic parachute, retro-rockets and a sky-crane. As Adam Stelzner has said, this technique skirts the very edge of what parachutes and propulsive landing systems can do in terms of braking. Curiosity was pushing the limits of the parachute/sky-crane system... Perseverance pushed almost to the end of those limits. The atmosphere of Mars is thick enough to be a nuisance, but not thick enough to be really useful. We might be able to build a supersized rover but we are not going to be able to simply use a scaled up version of this technique to land it on Mars, i.e. building a bigger sky-crane will not work, it will actually make the situation worse. If we try we will almost certainly end up with with a smoking pile of scrap-metal and electronic parts on the Martian surface.