I can only speak from a personal/subjective perspective of course, but once you're experienced enough (having heard a large variety of IEMs) and comfortable enough with your music preferences (you know exactly how you like your music to sound), it's quite easy to settle on an IEM/headphone or two and check out of the constant buy/sell/upgrade cycle.
Whether or not IEM tech has already peaked (I believe it has) 'endgame' is a choice, not a product. Once you've found something you can just plug in to your DAP and put on just about any song you enjoy with complete confidence that it will sound excellent, you can choose to be done with audio gear. If you can't, or get bored easily, you're probably just that type of person who gets easily bored, and needs constant change to stay entertained. And let me make it clear - there's absolutely nothing wrong with that! I'd also venture that someone who's never satisfied for more than a short while isn't really interested in how good the music sounds, but rather how different it can potentially sound with something new. Again, nothing wrong with that!
My personal goal is to find a combination of gear I can use to listen to my music as closely as possible to my personal sound preferences, and also use it to discover new music I like, without constantly feeling like I'm 'missing' something. Someone else's goal might be to listen to his music in as many different ways as possible, and keep pushing the bar as to how it sounds to them. If you're more like me, I believe 'endgame' is not that difficult to achieve. If you're more about new/different/better, there is no endgame - it's all about the game itself.