Which is funny because old schoolers (maybe even Gygax himself) hated what Diablo did to D&D.
No, not really.
We HATED that 3e was BASED on Diablo (and more so on Diablo 2).
Ironically, something very few of the hardcore 3e players caught onto or forgot when they went crying about 4e having ideas from WoW.
Whirlwind was such a direct nod to Diabo 2 among a ton of other items that you could find in 3e that many just shook their heads.
Of course, that same crowd also would say in many instances (and probably rightly) that 3e was a completely DIFFERENT game than what Gygax and Arneson designed, it was a game the MtG guys created and then bought the D&D name so they could slap the title onto their own home made game rules for their own RPG based LIGHTLY off D&D, and heavily off of Rolemaster.
The edition wars between 2e and 3e were FAR worse than anything I ever saw between 3e and 4e...but then again...
Over 24 million players drifted away from AD&D by the end of 2e.
3e brought in 5 million players (some say regained...but to the old guard who never came back...they'd probably say lost) but completely abandoned and lost 20 million players.
In comparison...3e to 4e only probably lost 3 to 4 million players total...small change to how many 3e lost.
5e has gained quite a gathering, regaining some of the old guard, but building a whole NEW generation of millions of players.
I think an edition change today akin to what 3e did, or what 4e did, would be the biggest disaster yet in regards to losing players. Good thing it's an update rather than a new edition, and it's going to all be under one banner (5e and the updated edition) of ONE D&D.
But yeah, it wasn't what Diablo did to D&D, but what the 3e designers used as direction and inspiration from Diablo to make 3e.