Wouldn't that imply that design is solved (at least regarding visual elements discussed here)? Then why not move onto other things? Why self-sabotage their success?

If I'm being cynical: because the design team at Apple needs something to do.

A lot of bad, unwanted features get written purely because "developers need something to do" and the same thing happens on the design side. I spent over a decade as a developer at various companies fighting bored designers who just had to redesign the look and feel of the app over and over because the current one was "stale" and "lacked pizzazz and pop!" But, then we devs would do the same thing to the feature list, refactoring and reimplementing and adding features for the sake of writing software, so... I was a hypocrite to complain.

That's probably not even cynical.

> I think a primary concern when Apple evolves their new design language nowadays is competitive differentiation.

Same as Google's Material Design. Both are bloody stupid, in their own distinct way.