I think with the Korean members it's more so JYPEs heavy reliance on them like 11 years as a group nearly and they're ridiculously active for that length of time because JYPE hasn't built an actual successor to them and even when it seemed like they actually will get a break then it turns out it's potentially only because the company are losing some of the members and would likely still just be working them to the bone otherwise.
Only member I'd really be concerned about leaving is Tzuyu because there is that element of if she does move back to Taiwan how easy will it be to get her to come back to Korea as that area of Asia is pretty notorious for idol members just not returning which even if it's not that extreme then it becomes how often can they realistically make a comeback under those circumstances
I was thinking about this last night, Twice has been JYPEs biggest cash-cow for so long I hope they are prepared for a decrease in steady revenue. Even if Twice stay together their popularity will inevitably wane as they reduce activity and age out. I've noticed JYPE is teasing a cute vibe for OBD which might indicate they want to recreate Twice's early concept. Hopefully this pushes them to promote Itzy and Nmixx better as well, Nmixx especially has a lot of potential for the future.
I've been mentally prepared for that because to me how hard they've been touring felt like a goodbye, not in a permanent way but like they knew things weren't going to be the same as before. If even a single member leaves JYPE I think that's the end of TTT an other in-house variety (at least with all the members) because it's just going to be too hard to get everyone together for none-essentials when they have their own schedules. It'll probably be yearly comebacks with some behind-the-scenes.
But the upside is that we'll probably see a lot more solo content from those members, looking at Red Velvet and SNSD many of those members went on to make regular YouTube channels.
It won't be the same but that's life, we're entering a new era no matter what.