I have been in kpop almost 4 years and I have seen many groups having ups and downs. By some reason big portion of groups are getting their best song almost always in the first two years. It feel like company is throwing all the aces during first two years and slowly after that the group is getting kind of mid tier songs just to keep their fans happy.
I think general strategy for companies is that you throw all the best song during first years. After that people get into the group and build their parasocial relationship to the group and accept what ever song the group bring to you later. Big portion of kpop fans praise their faves regardless what the song sounds like and they force to like the songs. That show how big marketing strategy parasocial relationship really is in kpop. Even groups that lost public attention, still keep selling millions of albums.
The first goal for companies is that you gain all kpop fans attention. Gaining attention from the nonkpop fan is harder task. Most of groups are selling pretty similar amount of albums. If you look big4 companies, most of them sell something between 1 - 2M(for ggs). It is pretty hard to sell drastically more albums than that. But if you save all your best song to the new group, you will have two groups that can sell over million.
I think companies are saving all their best songs and effort for the new groups, because older groups can not grow exponentially anymore and because kpop is very much limited between kpop fans and you have already gained their attention.
I think main reason why the group like Gidle have so much better longevity than other groups is that they make their own songs and they are not dependent by the company.