Sure but that's a compiler bootstrapping problem. It doesn't answer the question: why do you need to bootstrap the toolchain to build the distro? You can reuse a trusted toolchain that's been safely bootstrapped .
Because no other trusted toolchains exist under a threat model that trusts no single person or computer. We -are- the trusted toolchain.
If you cant trust your own machines outputs to build the next version of your code why should any user trust them
No user should trust them. If someone threatened me to put in a backdoor, I would. But it would not matter. Trust happens when many independent signed builds from reputable people all get the same result. That is why we make it easy and inexpensive for users to build the whole world from 0 from source at any given commit.