The issue is that the main case for why this feature is needed is a very close overlap with the cases where it is very questionable due to the reasons mentioned above.
As far as installing multiple compilers, go read Rob Landley's notes of the fun he has doing cross compiles, specifically all the work he has to do to beat gcc into using the right version of things. It sure doesn't look like it would be nearly as trivial as "install multiple copies of gcc in different directories"
If I had to do that, I'd use debbootstrap to create chroot sandboxes with different versions installed in each one. You don't even need containers, let alone VMs.