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.
Every mobile device sold in North America is unlocked for carriers. That wasn’t the case back in the day. Also locking down macOS has been for security. It’s way ahead of other operating systems for sound and app security.
Carrier locks have nothing to do with security or walled gardens. Two things using the word 'lock' is mostly coincidence.
And getting rid of carrier apps was a positive but it was moving from one walled garden to another.
And that happened a very long time ago. It provides zero defense against accusations of apple changing for the worse.