I really like the idea of Fil-C for derisking legacy code, but
- It's not an option for debian core infrastructure until it supports at least the same platforms debian does (arm, riscv, etc) and it currently only supports x86_64.
- It doesn't turn C into a modern language, since it looks like there's active development here getting the productivity benefits of moving away from C is likely still worth it.
If C isn't going away yet, just doing filc for x84_64 might still be a big win as it could catch a lot of stuff at runtime for x84_64 users that can then be fixed for everyone.
Given that a few languages from similar timeframe are still around, even if with fewer users, it is guaranteed to stay around as long as current computing models stay relevant.