That seems to be more of a testament to how awful bombards are, then anything else in particular.
See the secret to making your cities impregnable is actually to maximize melee/ranged unit strength. This is because your cities take on the stats of your strongest unit. So getting a caravel early for example, will boost all your cities to 40 str, and your enemies will have a hard time taking your cities, walls or not. Walls of course help, but I'm saying they're not really a big deal, at least considering AI incompetence. This is why AI civs like China/Brazil are harder to invade if they get their UU. Once they get their UU up, their cities start decimating invaders like nothing.
I mean, we have a mountain of evidence through people finishing the game really fast with like 40 cities or whatnot when the ADs roll along to show that the AI does not fail because the human turtles in their cities but rather that they are utterly incapable of stopping the human player's offense despite their massive advantages at high difficulties. And also the AI seems pretty adept at fighting itself and making headway in those wars, so to me at least, I really don't see how the AI would be more challenging if cities are weaker. That's also why I believe cities should be stronger, so war is much more costly and the human player needs to find other methods to catch up.
It does not help that the game overwhelmingly favors offense to begin with. More cities = better, and pillaging is seriously overtuned atm.