Apple is going to do the same thing they did with BSD, WebKit, etc. They will wait until proton is mature enough, fork it, then release it as their own. Why put in the effort this early on?
I think it's still hilarious and crazy that safari/chrome/webkit/blink exploded out of the cute little KHMTL browser called Konqueror in KDE from back in the day.
And the root of the whole browser wars thing was microsoft making an absolute dog of a browser for Mac OS X when it came out and then refusing to support it. lmao.
Usually I would be as optimistic as you are about this, because that would be the dream (although it would be nicer for them to contribute to the project.) However, given Proton's primary use case is gaming, such an effort will almost certainly be kneecapped by Apple's historic half-hearted commitment to anything other than microtransaction-powered mobile games.
Apple literally already has released a game porting toolkit which is basically Proton (see sibling reply)
https://developer.apple.com/games/game-porting-toolkit/