Blackpink doesn't have all of their solos set up (Jennie's is, Rose's is coming, Lisa's coming soon and Jisoo's is most likely coming in 2022). Their contract expires in 2023. By the time it comes, I think Blackpink would be able to do what they want.
BTS, they have the military thing but if they win Grammy's then they can be exempt (is that right?) but the competition is going to be hard and if some members go into the military, I have no idea what it's going to do to the group itself. But they debuted in 2013, did that mean that they resigned their contracts because 2020 would be the end, or did they have the 10-year contract? But they don't have anything set up either I'm hearing? Isn't Agust D Suga? I think he's a soloist.
Twice has absolutely nothing set up for them. No solos, no subunits, but they aren't really in danger. There hasn't been a huge scandal and females aren't required to go into the military for a while (they can if they want to though). There are not many things they can set up because their contracts expire next year. They could probably throw some solos in, maybe a subunit, and throw someone into modeling and/or acting. There's not much they can do. If they do decide to leave then they have almost nothing set up so they almost have to...
BTS renewed their contract so they have till 2026 on their current one. As someone mentioned before, I don’t think they will ever disband. They’re stockholders in BHE so they will most probably continue doing what is good for the company. Maybe solo projects for the members while squeezing in an OT7 album maybe once a year. They like the company and love each other, so why not continue working with people you love? So solo projects (be it music, acting, hosting, producing, etc) but still retain the BTS agenda going.
I don’t know about BP and Twice. Twice will depend on how many members will want to renew (or not want to). Each member don’t really have a stong enough individual fandom that will ensure their survival post Twice (or maybe just 2 or 3 members). They should work on that during the 2 remaining years they have left from this contract.
BP, on the other hand - each member is strong enough (in my opinion) to stand on their own. Renewing or disbanding would depend on their collective willingness to continue or go their own individual paths.