Just because bighit debuts many groups it doesn't mean their existing ones will be neglected. Each of the groups have their own individual management team. Not only but most of the groups aren't even directly under bighit meaning that their perspective companies manage them

Seventeen, Gfriend, Nu'est are still managed by their own companies. I-Land + the new girl group will be managed by Source + Belift - Bh is only responsible of their music. The upcoming bh group will debut in 2022 - enough time would have passed since txt's debut

Bighit is a huge company now. They have enough employees, their financial status is high enough to support all the existing groups under their labels. If they weren't capable of doing it, they wouldn't have even gone for it at first place