One is that funded software is almost always going to be more polished and easy to use than unfunded software. The vast majority of successful FOSS projects are either funded by corporations directly or by nonprofits which usually are still getting most of their contributions from corporations. True volunteers have no incentive to fix your bug or provide support.
Another is software provides diminishing returns the more it’s invested in past the point of viability. That’s partly because 95% of people need from the software comes from a small set of important core functionality, with the remaining 5% coming from everything else, but the 5% differing from person to person. Chasing that last 5% is not only very expensive and time consuming, it adds a lot of complexity and overhead. So there is a lot of FOSS that only solves up to the 95% point because it’s not too difficult and the people maintaining the FOSS have a different 5% than you do.
Anyway, it may not be the case that all software will be FOSS any time soon and GIMP will replace photoshop, but I certainly wouldn’t consider GIMP a failure. If you just need something basic and free for some light editing it works fine. It’d be a shame if people with simple needs had no free alternative to expensive licensed software.