Nothing is fun forever - its why TV-series have more than one episode, why you dont read the same book over and over again and why most people tend to play more than one videogame. Heck, its the reason I cook a different meal every day because yes, pizza is very tasty, but eating it every day doesnt make me happy.
I'm sure you're just trying to make some weird point about this content being "bad", but at least in my book thats not necessarily the case - if people wouldnt move on from content, there wouldnt have been a need for any expansion or patch whatsoever. The dungeons in ARR are supposed to be fun content after all - why did we need anything beyond Sastasha?
While I'm personally not a fan of Bozja (specially because the areas looked boring to me - I get why, but that doesnt make it anymore appealing), but I still spend quite a while with that content - probably a lot more time than I spent with a lot of (shorter) singleplayer-games these past few years. I dont see a problem with coming to the conclusion that the content was genuily fun for the first 15 hours, followed by a grind for the next 40 (numbers might not be accurate, I didnt track how long I spent with that content).
I dont expect a single piece of content, a single game or a single meal to sustain me forever - I'm not that unrealistic.
Honestly: My big hobby next to videogames is cross stitching, which involves me doing the same motions over and over again for hours. Often I dont even care much about what I stitch, because its just a nice thing to do while watching a netflix show. I still prefer doing different patterns instead of stitching the same thing over and over again - even though the stitching itself is fun for me, not the outcome. So you could argue "Why do you follow a pattern at all if the process itself is supposed to be fun?"
