1. Make money while preserving the UI
2. Make money while destroying the UI
... then why would anyone bother to preserve the UI? It's completely irrelevant even in the best case scenario. But allow me to present another more cynical:
They both hate anything good/competent/well-made, and the users too, and also want to make money.
> In fact making the UI garbage will chase away users
When I first started reading reddit in early 2006, the web was a larger place with many thousands of fora. But reddit's growth was at their expense, most are dead or dying. It's depressing when you're looking for something specific, and you end up on one of the old phpbbs... no one's bothered to post a comment in months. If they have, no one ever replied. But in their archives, it shows that just 15 or even 10 years ago, an hour wouldn't go by without someone saying something.
They've solved the "chasing away users" problem by making certain there's nowhere to flee to. Some small fraction might just cut that sort of internet use out of their lives entirely out of spite, but not enough to matter.
In fact making the UI garbage will chase away users and will make them less money in the long-run, or maybe even run the company into the ground if people starts to migrate to some alternative with non-sucky UI.
There's also nothing about the UI that's related to AI.