Over the past decade, there's been a lot of regulation forcing Apple to open up their "Apple only" integrated platforms.

It used to be the case that if Apple wanted to build a walled garden / cathedral, then in order to compete in the hardware marketplace they had to provide software that didn't suck. You knew that if you bought an Apple product, there was reasonable assurance that everything was tightly integrated. If it wasn't, you'd go buy a market alternative (Android, PC). In my mind, this means that they spent a lot of time and dev resources (i.e. money) on their Frameworks. I think it showed. Time was spent on design. They focused on opening up capabilities "the right way."

Now that's pointless. If the iPhone is just an Android phone with a different coat of paint, then dev resources are going to be shifted to a place where Apple can distinguish themselves in the market, where they have platforms that they can control: Services.

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.