I agree that sandboxing should be decoupled from the distribution platform. Unfortunately, normal users aren't going to use bubblewrap or firejail directly until better GUI wrappers are made.

I also think that sandboxing is still a worthwhile pursuit for many FLOSS packages, since many programs--from moddable games to web browsers--run untrusted code. It also mitigates damage done by some forms of user and developer error (can't accidentally wipe a homedir if you can't access it).

It's similar to the rationale for not logging in as root unless necessary.