I'm sensible to the issues of tradeoffs (e.g. platform support which is a completely fair issue) and manpower requirements and whatnot, but I don't see why it can not switch language. Converting libraries "inside out" has been done. Adding Rust support inside sqlite's build system and migrating modules is technically feasible (again, not opining on whether it would be worth it).
I mentioned that but i don't think that's the subject as it does not warrant such a drastic assertion that sqlite can not switch language.
There's no particular reason to think that this will ever happen. SQLite runs on a lot of crazy stuff that Rust won't want to bother with.
If can switch the officially endorsed "primary" implementation, but only after an alternative implementation has been around for a long time and was battle-tested, all the while the original C implementation continued to exist and develop.
At a purely technical level (ignoring issues of platform support and all) it pretty much can do that, actually: integrate XYZ into the build system, build new features in XYZ, start converting old features to XYZ, end up with an sqlite in XYZ.