Yet by the active unique user numbers they have estimated sold at least ~168k copies and maybe upwards ~500k https://steamdb.info/app/280680/charts/ And that's Steam alone. I'd be really curious compared to this how many people click on the Donate button on the Krita homepage.

That's a lot of purchases imo for a project like this. I'm kind of ambivalent about the store cuts but they give a huge enough exposure to any game or software.