Even if Mistlands was released today you would have people spend only so much time before whining and complaining for more.

If you are worried about burning out from playing Valheim - find other games while waiting for future content. This is a small indie company that took whoever know's how long to get the base early access product that people then had the chance to invest in. No matter how successful they became for said game - that doesn't equate to major increases in speed when it comes to development and Valheim did manage to hire 1 or 2 more developers who then would have to be trained to work on said game with said team.

You may love this game but to me you don't really know how to approach an Early Access game. It's not like we have foresight to know whether Valheim will even release or not, how often content is dropped and so on. You either invest into the early game and choose to play or not.

Groaning and moaning about development pace will only put you in the camp of those who continue to beat on a dead horse thinking that they are performing some version of cpr or necromancy. Valheim also doesn't really require a large community of constant players unlike some sort of pvp game or "live service" game. People leaving is actually probably a good thing because then they can come back fresh as opposed to completely burning out.