Or maybe GIMP can do everything they need so nobody needs to get paid. But if nobody gets paid anyway, might as well copy photoshop instead. Legal issues aside, it doesn't seem that irrational.

I think in practice, the moral argument just doesn't work too well for intellectual property, and creators need to deal with it. Which, unsurprisingly, they do, in different ways (be it through making payment convenient like Netflix, prosecuting individuals like MPAA, or focusing on enterprise deals like Adobe, ... or still pushing the moral angle because a statistical effect is better than none).