Can you support this unfalsifiable reasoning beyond blaming a convenient political scapegoat? Which paragraph of which article of which regulation requires them to deliver low-resolution PDFs in Apple News, for example? What about all the other issues?

Your argument essentially boils down to: If Apple doesn't get to do whatever they want without compromise, their execs get too discouraged and depressed to innovate. The obvious conclusion is that the only way we can enjoy the unrivaled genius of Apple is to give them a blank check to do whatever they want.

Every act of consumer protection and every form of pro-competitive regulation is twisted and exaggerated, no matter how insignificant it is to their bottom line or product functionality. The world is ending any time they don't get their way and when the world doesn't end, this decision becomes the scapegoat for all of their future faults, missteps, and bad performance. They can never do anything wrong and nothing is ever their fault, it's so so incredibly tiring to listen to this.

Which regulation made Apple News have low res PDFs? Which regulation made the search boxes in Liquid Glass transparent and show text from the window behind?

The company as a whole has changed across the board.

The beginning of Apple’s backslide far predates any (thus far fairly limp-wristed) attempts by regulators to pry open their iOS walled garden.

At least in North America - their biggest market I think? - the iPhone is still utterly locked down. Far more locked down than, say, their Macs were when OS X was at its best. Meanwhile macOS continues to get more locked down and yet still worse. Your theory just doesn’t match reality.