That’s exactly how flatpaks work if the library you need is not in the runtime. Which is very often the case.
I know because I made one for my personal use and the package was not available elsewhere.
Additionally, at least for Flathub, they have shared modules for commonly used libraries that aren’t in runtimes. (Many are related to games or legacy support like GTK2.)
So we’re just reinventing the wheel with more bloat? Brilliant.