And Apple is no better in this regard. Both companies obviously want to prevent sales outside of their walled gardens -- soft deprecating independent software with warnings and 'on by default' blocks.

Of course they do, why not take 30% of everyone's work and keep control?

Maybe I misread Krita's situation but I'm not sure why they don't just pull out of the store and sell independently. Plenty of payment processors would be happy to take them on.

Pricing is always difficult, but for a product that well known there's almost certainly a tiered system they could set up that generates enough income to keep the lights on (or better).