Technically isn't D3 already sort of dead? It's player base continues to dwindle and the word of mouth is pretty bad right now. (I like the game, I'm not a hater)
I don't think PoE is meant to be a "D3 killer" anymore than LoL is meant to be a SC2/WoW killer. It's a f2p game that gives people what they want, and will tempt players by hooking their friends first. Nearly all my SC2 friends are playing DOTA2 or LoL, and don't seem interested in HOTS either. So far it's the completely the opposite reaction I've seen in WoW over the years. Whenever a "WoW Killer" came out my friends would get a bit hyped about certain aspects of it (pvp mainly), play the demo, play the final product...and quickly come back to WoW. Even now I haven't seen many people stay with GW2 or the f2p Star Wars - they come back to WoW or stop playing MMOs.
It's not really anecdotal with popular f2p games right now, they're starting to dominate a lot of PC gaming. SC2 has lost a lot of relevance lately and I see parallels with D3. The hardcore SC2 crowd isn't as hardcore anymore overall, there's a lot of complaining about the metagame and design decisions, etc. But perhaps the biggest thing is the game's difficulty curve turns casual players off, who prefer playing team games with friends (hence, LoL). D3's player base is also suffering from disinterest from a lot of casual players, and hardcore players complaining.