This is nice, but what are the actual benefits of “bootstrappable” implementations?
As a user, bootstrappable implementations, together with reproducible builds, provide confidence that you are running the code you expect to be running. Its source code is auditable by the developer community, which in turns provides reassurance that the code you’re running does not have backdoors.
Bootstrappable implementations provide clear provenance tracking: the dependency graph of your distribution packages shows how each binary was obtained.
Having an automated process for bootstrapping your distribution on existing architectures greatly simplifies porting to new architectures. With a formalised system bootstrap process you can just focus on issues specific to the new architecture.
If you are a compiler writer, making your compiler bootstrappable from a different language will simplify the development process (no need to carry large pre-built binaries around). It will also make it easier to port the compiler to a different platform for which no bootstrap binaries exist yet.