Not that weird behavior IMO.
Honestly, many fans act like such retards sometimes, they complain that album X sucks shit, or that "the band hasn't done anything good since album Y" and similar things, it can't be easy for artists and bands to hear that kinda of criticism either.
Sometimes it doesn't even matter what they do, if they do something one way they'll hear criticism from some fans, if they go another way they'll get shit from other fans.
And in this example, I think Daniel gave a good answer to the facebook-post.
Yeah, two members are leaving, but is he supposed to disband Pain of Salvation completely? No, instead they are moving on, and I think it's a good thing personally.

The problem with many fans complaining is that it's very hard to please them, calling PoS a Daniel Gildenlöw project because two members are leaving (and have to be replaced) is one thing, but if he had ended Pain of Salvation instead, they would have complained about that instead.