Thinking pragmatically, the legacy systems where there is no current rust toolchain most likely do not need the drivers and components that are being written in rust.

Unless you somehow want to run Apple M1 GPU drivers on a device that has no rust toolchain ... erm...

or you want to run a new experimental filesystem on a device that has no rust toolchain support?

The answer to the "new and emerging platforms" question is pretty much the same as before: sponsor someone to write the toolchain support. We've seen new platforms before and why shouldn't it follow the same pathway? Usually the c compiler is donated by the company or community that is investing into the new platform (for example the risc-v compiler support for gcc and llvm are both getting into maturity status, and the work is sponsored by the developer community, various non-profit[1][2] and for-profit members of the ecosystem as well as from the academic community.)

realistically speaking, it's very hard to come up with examples of the hypothetical.

[1] https://github.com/lowRISC/riscv-llvm

[2] https://lists.llvm.org/pipermail/llvm-dev/2016-August/103748...