Then I believe I am even further from understanding what point you're making.
...
Even just focusing on the first 3 Sonic games (& Knuckles included) each game grew exponentially with new features. Is the way SRB2 handles shield abilities and two-button controls really that wild a departure from the design ethos presented in the Classic series' evolution?
I'll try to remedy!
I don't think two button is a wild departure. In fact, I think it's logical!
If you're going two-button, using the second button in this limited way seems like both (1) a half-measure; incomplete (2) a peculiar deviation from the shield functions as established in every other version of Sonic, at least classic.
In fact, SRB2 adding "Jump Spin" abilities would
better follow that exponential progression of abilities you outline from 1-2-3K, the primary difference being that those are designed around "1 Button gameplay."
If the goal is to mimic "1 Button" as close as possible while adjusting for 3D, on the previous page I pitched a version for how SRB2 / 3D Sonic could potentially work on one button; if it follows the classic shield functions, then it's just about how to solve the SA issue of crouch/roll.
(Movement Back + Jump would become roll, or crouch if starting from stationary; MB+J+J would initiate spin dash)
I'm not saying that's better than 2 button, I'm saying it's truer to the aim, if the goal is to be as to-the-book faithful as SRB2 positions itself in other design decisions.
Hence, I think adding native "Jump Spin" ability simply makes the existing controls and design
more intuitive,
more balanced in favor of the player than the layout, and
more consistent and fluent in translating the spirit of Classic Sonic to 3D.
It just seems very... un-Sonic-like for shields to have a unique input. If there's a unique input of control for the character, it should work regardless of whether or not a shield is equipped; it should do something. The shield can change what that ability does. The shield is like Popeye's spinach, it's not, like, giving him a third arm he didn't have before, if that metaphor makes any sense.