I think I understand your point, but I feel like it's just that time did his thing. You got older, maybe your taste slightly changed, maybe you got very familiar with the core system too fast and are getting bored of the game already and/or maybe the pace of the patches/game doesn't suit you anymore. It is something that is bound to happen but takes different time for each of us.
And I always feels a bit sad when you realise you don't enjoy a game as you used to after a while.

But it also has to do with how games evolved with time. 15-20 years ago in MMORPGs, it used to take months if not years to reach max level, and after that you had to spend at least the same amount of time to get all the pieces of the gear you wanted. You also didn't have access to as much information as we have now on the internet. It was common to see some strange build appear with more or less success on specific scenarios. (how much fun I had with my battle priest where the goal was to drag the fight to kill the opponent slowly while healing myself instead of the usual full magic attack ones where it was only a race on who can kill the other faster)
Time passed, and the leveling process was done shorter and shorter as the majority of players found it boring (which in most game was the case for me too but the ones i enjoyed the most were actually the ones with the more interesting leveling process). The process to get the equipment was also shortened after that, so lifetime of games end up shorter as people want more things faster.
At the same time, solo game went from games where we had a clear linear path from A to B to a more broad system like open world where we got rid of B but you can do whatever you want. B technically still exist as it's now up to the player to define where it is now.
That last part is slowly making its way to MMOs too. We want more different things to do and we also want to do them faster.
Of course that is not the case for everyone. and we live in a time with a huge market, so most of the time it ends up as simply finding the game that suits you better (which is what the "go play something else" should refer to and are sometimes a bit too rude).
It is understandable to want to change something you don't like, but there are also other who like what you don't like. So in the end it's up to the dev to choose which side the want to focus first (because it's not impossible for them to come with something that can satisfy both sometimes).