2022 is late for Diablo 4 release. Its engine isn't that good to face the potential new aRPGs being developed atm. If they delay D4 to 2022 they'll make a mistake. No matter in what state the game is, it has to drop next year or they risk getting smashed by better games and lose a big portion of the player base they aim to get.
The truth is Diablo 4 is a great game for 2020, but it would be mediocre in 2022 since the games market is so dynamic. Diablo 4 has to capitalize on what it is in 2021. That's my personal opinion.