Just like D2, players who don't enjoy repeating the game to improve their characters will leave and those who want to devote the time will stay
But unlike D2, D3 has a lot of competition and it's unlikely it'll get anywhere close a similar cult following
You asked what is pay-to-win in WoW, and there was a quite clear post on that
There is a PvE "race" in WoW, that D3 doesn't have. It's mostly coop, so buying gear has no negative impact on others... well I'm certain there will be complaints unless Blizzard adds PvP with those with RMAH items separate from those who doesn't, but yeah.
I could just imagine if WoWs most hardcore guilds could buy BiS items and just fly through content. Even regular guilds asking their members to buy trinkets and weapons to speed up progression..
Nah, in WoW this system would be utterly destructive to the game