I swear, most of these arguments against a truly happy ending for Karlach make even less sense than the entirety of Act III... I can't help but agree with you, that these arguments exist for no reason other than pure spite...
I mean, why doesn't Gale also just blow up? Ideally, immediately after the BBEG is dealt with, the story's settled, and the tens of hours of game time spent trying to avoid that fate for him through direct player intervention, just because it would be more flavourful if Mystra turned out to actually be a vindictive douche...
I'm being sarcastic, of course, this entire thing just... blew (pardon) my mind...
