The progress up until the ending is what should be emphasized.
You dont need a bunch of progression after the ending. Since it has ended.
Which of course does not mean there should be no progression. Finding somewhat better gear offers that progression.
And you can have progression like increasing your MF etc. a bit.
Just not infinite power, or anything resembling infinite (such as P20000).
We live in a weird time where some people want to skip the actual game as fast as possible, so they can have some number go up after the game. I bet they are a small minority. But regardless, Blizzard should ignore them.
PoE does not have infinite power scaling afaik? Nor 50000% dmg affixes.
Neither does most other A-RPGs around.
I bet the genres essence will be just fine without being D3.