I expect D4 to come out Q2/3 2022. I would love to see a Q4 2021 release, but Blizzard themselves said at 2020 that it isn't even Blizzard soon. Outside of some very base concepts we've hardly seen anything about the game, I expect to see a lot more information at this years blizzcon, a Beta announcement at next years Blizzcon and a mid 2022 release.
The public being able to play a demo that show-cases skills, talents and game-play with 3/5 classes as well as more in-depth info about gear stats, lore and monster families is "hardly anything"? lol idk why you guys are so jaded about how much we've seen and how it seems like its "not much" when its literally 80% of what we'll experience at launch. Whats left to reveal besides the last 2 classes that we already know are amazon and paladin, crafting and end-game systems? And we'll get most of that info soon. Its an ARPG, not that complicated of a game with not that many systems.
We have the Dev update in a couple weeks, another in a few months and then online-blizzcon for more in-depth info. Blizzard not releasing every detail about the game immediately doesn't indicate a super-far release because that's why they set up these quarterly-dev-updates in the first place. Withholding the info on purpose to slowly release and keep the hype-train running. Most of their feedback and changes will come with Alpha and Beta testing at this point. They've already gotten the general public opinion.
I think there is room and a fan base for both games. A lot of hardcore poe players will never be satisfied with Diablo and vice versa.
For me, there are just too many systems in poe, crafting is (for me) a nightmare and I can't even start to understand stats and gear, having said that, I have spent a lot of hours in poe and had fun, but it will never be my go to game.
Of course POE will always have some sort of fan-base. I'm just saying that its fan base is extremely tiny in comparison to Diablo and will shrink tremendously upon D4's release. It only ever had an inflated fan/player base due to waiting periods between D2-D3 , D3's crappy gameplay sending players back to PoE and now waiting for D4.