As many have said there is a difference between gg and bg. I feel like the 4th gen gg era truly just started and the shift is happening. In a 1-2 years a full shift will happen, leaving maybe one or two 3rd gen gg staying relevant at best. For bg, it's not as clear as they always have better longevity compared to gg.
It's just a cycle that is inevitable... idols want to eventually get married, have a family, have freedom and be away from the public eye eventually... Companies also tend to invest more on the newer groups and invest much less on older groups. Companies also make most of their money from groups in their first contract. Typically if groups renew (very few of them), the contract becomes much more favorable to the idols than the company.