It's called the Left-Hand Wall rule, and while it does argue that the most complex organisms will typically become more complex over time, it certainly does not argue that the average complexity should increase, or that any individual line will become more complex.
The first caveat is where most people fail. They assume that "complexity increases over time" means that complexity as a whole, or on average, or something, increases. That's certainly not the case. While the Left-Hand Wall leads to a small increase in average whatever-value-is-being-looked-at, the thing about this model is that the overwhelming majority of entities remain very low on that scale. You get a population that's so right-skewed that it's not a bell curve, but an arc. In fact, that may be the best way to visualize this: Take a pen and paper, and draw a graph. X is "complexity", Y is "number of species". At T=0 (when life arose) X=1 and Y=100%. At T=2, maximum X=2, at X=1 Y=99%, and at X=2 Y=1%. At T=100 maximum X=95 or so, and at X=1 Y=85% or so.
These numbers are coming out of thin air, but it'll give you enough to get the idea--the vastly overwhelming majority of creatures stay near the Left-Hand Wall, and only a few break out into the rally high values.
The first caveat is where most people fail. They assume that "complexity increases over time" means that complexity as a whole, or on average, or something, increases. That's certainly not the case. While the Left-Hand Wall leads to a small increase in average whatever-value-is-being-looked-at, the thing about this model is that the overwhelming majority of entities remain very low on that scale. You get a population that's so right-skewed that it's not a bell curve, but an arc. In fact, that may be the best way to visualize this: Take a pen and paper, and draw a graph. X is "complexity", Y is "number of species". At T=0 (when life arose) X=1 and Y=100%. At T=2, maximum X=2, at X=1 Y=99%, and at X=2 Y=1%. At T=100 maximum X=95 or so, and at X=1 Y=85% or so.
These numbers are coming out of thin air, but it'll give you enough to get the idea--the vastly overwhelming majority of creatures stay near the Left-Hand Wall, and only a few break out into the rally high values.