Planes in the air always need to apply some forward thrust; they slow down by applying less of it. So the effect on your energy budget is not even a potential net gain but a decrease in your net rate of loss.
How do gliders avoid that requirement? I don't think there is such a requirement.
Passenger & cargo planes carry a lot more weight with a lot less wingspan & wing area per weight at much higher speeds against much stronger drag.
To replace them with something that did the same job with a different power source, you'd still need to carry the same load at the same speed. To start simply, let's skip speed for a moment and just look at load. A 787 carries 423 times as much weight as a DG-800, for a total weight 380.7 times as heavy when the weights of the vehicles are included, but it does that with only 32 times as much wing surface area to generate lift with. That's about a factor of 12 as much weight per wing surface area, so it needs another lift-generating factor in its favor to help it generate roughly 12 times as much lift as it could generate if it were a 787's size but trying to operate like a DG-800. That other factor, without somehow giving it 12 pairs of wings or making its wings 12 times as wide or such, is more forward speed. Conveniently, the plane's forward speed is one of the two requirements we started with in this paragraph's first sentence anyway.
So, let's design this thing for more forward speed. Well, that means running engines.
How much speed difference? The world record for fastest that a specially built-for-speed glider has ever even briefly peaked at was 180 MPH. The DG-800 I used in that comparison above was one of the fastest, with a top speed of 170. But including the whole time from the beginning of a flight to the end so the ups & downs cancel out, any enclosed glider typically goes at about 50-60. A loaded 787, under most realistic atmospheric conditions, can't even get/stay off the ground at less than roughly 140-150, and typically cruises at over 560, possibly up to almost 600 in a pinch. That means fighting a wind of hundreds of miles per hour in its face the whole time. One of the most basic flight equations is that if you're maintaining speed & altitude then your thrust equals your drag, because if thrust were greater you'd be speeding up or rising and if drag were greater you'd be slowing down or sinking. So keeping a vehicle that size & weight going means maintaining thrust equivalent to the drag being caused by that amount of wind in its face. If the thrust were to go to zero (actually a negative number if you start trying to "regenerate"), all that drag would make speed and altitude get very dire very rapidly.
Less/no thrust, at any time during a plane's flight, means much much less speed or lift or both, which means simply not doing the job. Doing the job means maintaining thrust at all times. There's no way out of the basic physics. Gliders, as you put it, "avoid that requirement" by simply not having anywhere near such a demanding job to do: carrying a tiny fraction of the load at a small fraction of the speed.
Another way to look at it that's a bit word-gamey but a fair enough simplification: regenerative braking is a type of braking, passenger/cargo planes don't brake in flight or even have flight-brakes, so there's nothing to try to apply the word "regenerative" to.
It’s a big heavy box with a positive and negative terminal. It’s electricity. Unlike a giant vat of liquid you can put it wherever you want.
You have the difference between a fuel tank and a solid shape exactly backward. The fact that fuel is a liquid is exactly why you can make a tank in any shape and put it wherever there's some leftover space space for it that you can't put something else. That tends to end up being the insides of the wings & tail fins. (This is especially easy to see in a diagram comparing the A, B, & C models of F-35. What goes in the extra wing space that the one with the biggest wings has and the others don't have? Fuel. What goes where the lift fan would be in the ones with no lift fan? Fuel. What goes where the built-in gun would be in the ones with no built-in gun? Fuel. Fuel tanks are what you use to fill the spaces that aren't needed for something else.)
Fortunately for the battery-replacement concept, you're also wrong about the shape of batteries. They're really paired chemical tanks with some wires in & out of them, so, again, just like fuel, you could make the tanks just about whatever shape is convenient.
Unfortunately for the battery-replacement concept, you're also wrong about what their solidness means for replacing them. Being solid, even in the most ideal shape for the given application, makes replacement harder to design for and harder to do, not easier, than with a liquid.
Well I guess you could design a plane where the batteries are hard to get at as easily as you could a car. You could also design it to be really easy.
Yes, it would be easy. It would also mean the plane had no baggage space, because the place where replacing batteries would be easy is the bottom of the fuselage, which is where baggage goes in liquid-fuel planes. It would also add lots of weight to the plane, not only because batteries are just heavy anyway (which also makes battery cars heavier than liquid-fuel cars), but also because of structural reinforcements that would be needed to handle the attachment & detachment mechanisms and make sure that both parts are structurally sound enough both separate and together. And those reinforcements would also cut into the cabin space as well.
You could instead keep the baggage space and make the batteries the shape & size of current planes' fuel tanks and put them where the fuel tanks currently are, which is primarily in the wings... but that's the most horrible place you could come up with to be trying to attach & detach solid objects. They'd instantly become the weakest, least-stress-tolerate points in the whole plane's structure. Even lots of reinforcement all over the place would only eat into your battery space and add more weight while still not really fully solving the problem of how badly you'd just compromised the structural integrity of the wings. You could avoid this by making the batteries a more permanent fixture as part of the wings, but then once they'd been used what would you do? If only electricity were something that could easily flow in & out of any shape without needing to physically move the container anywhere, almost like a liquid...
Batteries in a plane could power propellers, no problem, but for large-scale commercial flights would that be enough? If not, what would provide the motive force?
A passenger/cargo jet engine's thrust is about 30% from exhaust gas rushing out the nozzle and 70% from the main fan at the front of the engine, which is driven by a turbine (like a small portable windmill) that takes its power from the exhaust gas on its way to the nozzle. An electrical system could spin a ducted fan easily enough. I'm not sure what it would do about the other 30%. Spin the fan 43% harder?
