To me it's not really a matter of competition, as PoE VS Diablo looks more Apple VS Oranges the more time passes. I mean, it's obvious they're competitors in the genre but given the widely different design choices i think they can just coexist with not many issues.
What makes PoE important for Diablo, ARPGs and the industry in general, is how GGG is doing things. They have first of all a f2p game, and this attracts a lot of players. They are completely transparent in communication, are not afraid to admit errors and to try new stuff out. They make choices in design and know that this approach can drive out some players but they stick to their philosophy.
Basically, they never tried to do an AAA game, they just try to make a good one. And this is paying off especially since old big studios have morphed into cash grabbing machines devoid of any passion.
I am not hoping for Blizzard to change. I am hoping for the development team to have enough room to work with D4 and actually create good gameplay instead of being governed by white collars aiming for max profits with the minimum investment.