The distribution provides support for much longer than upstream does. Also, packaging is actual work. Just because some hipsters in some company consider it cool to release new features every 4 weeks, it does not mean that the volunteers of some linux distribution can keep up with thag. So if you want your distribution to work well, you should focus on workable and transparent interfaces.

If you are happy with the support cycles of your upstream and can live with a black box, on the other hand you don't need a distribution in the first place.

Besides, the whole "vendor everything, link statically" idea works well only for the leafs of the tree. Guess what Rust would do if llvm was not a readily usable library but a bunch of C++ files directly used inside clang? Thanks to the Rust mode of operation, it is impossible to do with the rust compiler what they did with llvm.