"Can "most people" afford paying $600 yearly fee to edit images?"
Considering Adobe's Creative Cloud is pretty popular, it seems that yeah, they can. But that's irrelevant to whether a piece of software is FLOSS or not.
"although they may not know that these problems are caused by Adobe apps being non-free"
How dare those bastards ask to be paid for their work!
"I just don't think that suggesting proprietary alternatives in this context is particularly helpful as they also lack features of freedom, e.g. sharing, privacy and absence of vendor lock-in."
In this context, nobody gave a shit about that. They wanted the features that let them get their work done. If FLOSS doesn't have it, it doesn't get used. End of story.
"If Adobe freed the source code of Photoshop, how long do you think it would take for missing features to make it into Gimp?"
Irrelevant, cause that's not going to happen. If the Gimp developers aren't good enough to make those features themselves, without copying off of Adobe, perhaps they should work on something else.
"... except if the task involves working with other people who may not have access to that software, or may not want to run proprietary software, or may not want to give up their privacy or freedom."
So, nobody? Photoshop is the industry standard. If you're not using it, you're the one who's the odd person out, not them.