being adopted by much more artists doesn’t determine the software’s quality
It does determine the software’s quality long term.
FireAlpaca isn’t open source, Krita is. The more appealing Krita is for the average artist means more users, more users means more contributors like me who are willing to put the time and effort into making plugins and even contributing to the codebase to add features: Brush Preview - Feature Request - #71 by tenzindraws
Your point would make sense if Krita wasn’t open source but that’s not the case. Krita benefits immensely from more user adoption than any other painting program since its development is openly collaborative.