yeah making groups generational is probably the easiest way to fix failing groups or to patch holes after grads, but it often comes at the cost of the group's identity and also altering the group dynamic. but at the same time, as much as i'd like to see more stable groups, i think this is more of an adaption to the current idol landscape. i mean turnover's always been high in indie/chika idol groups and members come and go all the time, and that's sort of leaked into more mainstream groups in the last decade, especially with the idol boom which was kicked off by akb48, a huge group where members are added and graduated fairly often. and i notice a lot of people bring up berikyu and how long they were together, but also i think it's important to acknowledge that those groups were formed before the idol boom, which made generational groups more common, and when the girls were super young; by the time berryz disbanded they were only in their early twenties iirc, and c-ute was the same i think. i can't speak for other groups from the 2000s not in h!p since i don't really know of many of them, but afaik in the last decade rotational groups have definitely become more common so to me it makes sense that h!p's kind of adapted to that and made most of their groups rotational. and also, rotational groups being a thing means members who don't want to go when their groups are disbanding can be moved to other groups. the obvious and only example for this is reirei, but i'm hoping that this kinda becomes more of a common thing since it doesn't mean that idols HAVE to graduate when their groups disband.
...all that said though, yeah i wish we saw more stable groups lol. like i said at the beginning making groups rotational means it kinda dilutes the group's identity, and also doing it so often means new members can't adapt since they don't have the time to (i mean j=j added yumeriai last year and they've only had one single with minimal attention given to them, and reirei hasn't even debuted with the group yet, though it's upcoming with the new single). idrk where i'm going with this but i guess like, as much as i want to see groups keep stable lineups, i don't think making all groups generational is much of a problem in the long run. and i don't really think this is something that'll exactly benefit j=j or tsubaki (especially the latter), but well, like megumibex i'm gonna be cautiously optimistic about it, because in the end it's fun to see new idols debut, but i think what we're all mostly worried about is possible graduations.
(yeah obviously now that i've had more time to think about it i've changed my mind from my last post here LOL. that was more a first reaction, and i'm still not really happy about this but like i said, i'm gonna try and be optimistic. which is hard since i'm super pessimistic most of the time but these are idol groups it's not that deep lmao)