Artists can be used for Golden Ages instead, as well.
That is true, but with every other GP, including musicians and artists, you are guaranteed the luxury of two choices instead of one practically at all times. For artists and only for them it can quickly get cut in half, pretty much reducing the utility of the unit for as much as well.
Additionally, because you have to deal with Artifacts as well as art, they are different than Music and Writing works already. Basically, it's a lot more acceptable for Great Works of Art to be difficult to have slots for if you don't have Wonders and the like because you'll get Artifacts later on anyway. And a lot of those. Plus the theming bonuses tend to be for Great Works of Art and Artifacts more than Writing and Music.
I don't see how the existence of artifacts helps great artists or great works of art. Artifacts don't directly supplement or have strong synergies with great works of art - you're better of theming your museums and wonders with homogenic pairs of either just artifacts or just art. In fact, I'd say the opposite is true - art and artifacts have to compete for the same slots, and since, as you said, it is easily possible to obtain a lot of artifacts, art is less desirable. Here's why: you always can trade creating art for starting a golden age, which is always useful; with historic sites though, for the most part, you dig in nobody's lands or in the lands of some civ you have already erected at least one landmark for, so creating an artifact becomes the only valid option.
Why can't there be something unique about one of the Great Work types? Museums as we know them today literally didn't exist during Medieval and Renaissance times. There weren't many non-religious organizations that would collect works of art to display for the public to view (=tourism). The Uffizi is an example of an older museum, and you can build that in the game if you want Art slots.
I agree with the historic premise, but other than that there's not much of unique about the great artist having less utility than musicians and writers. It's not as much unique as it is weak. If artist had something to compensate for that, then I, too, could be asking 'Why not?', but I don't think that argument is appropriate in the current situation.
The game gives you options, take them or change your gameplan if they become unobtainable. Every playstyle can't have everything. Should we complain that there's no good military units for a long time in the top part of the tech tree which hinders sciencey/growth/infrastructure -based civs from going domination? No, because if you want to go domination, you focus the bottom part that has better units and military focused infrastructure.
If you're saying that there should be diversity of gameplay choices, I fully support that. Choices are good, choices that provide a trade-off (top part of tech tree vs bottom; great work vs instant effect) are also good. I could also get behind the premise that you
have to plan which religion to found or adopt in order to have the right slots available, but I don't see why this has to only concern great artists. I find the discrepancy is unjustified. Sorry if I keep repeating myself.
Eventually, imagine if what I suggested in the end of my previous post was true (no great slots on amphitheatres or opera houses, religion is the source of slots). Creating a religion would become dauntingly trivial, as the balance would greatly tip towards religious buildings. You wouldn't even be able to cover all three types of great works - perhaps that's also planning for some, but I'd find it irritating. Going for a culture game would mean tying yourself unnecessarily tight to a completely different game mechanic, which is meant to be victory type neutral, - the religion game.