Yea I know if you wet it it will come down.
However if you wet it in small doses it actually becomes plyable. This is how we do barrel vault ceilings.
I totally agree you should just replace this and be happier with the end results. When you remove the ceilings you can put straping up(3/4)inch boards 16 inches o/c. Then you dont have this issue any longer.
If you don't like taping corners just install some moulding or crown at the ceiling and wall connections.
More bang for the buck when you sell.
No more band aid fixs....