Imo the situation is more like if overleaf were also the people who made the LaTeX project originally.
I think the only possible issue with the typst org dying (assuming after the full 1.0 version so it's mostly maintenance) is that packages are automatically downloaded from the typst site, but an open repo can trivially be made considering that the set of packages used is just from a open source git repo and the closed source site just hosts tar.gz files of the folders in the repo. Not a big deal I think.