It's always seemed obvious to me that it would be better to make C safer than it would be to rewrite the billions of lines of C that run all our digital infrastructure. Of course that will get pushback from people who care more about rewriting it in a specific language, but pragmatically it's the obvious solution. Nice to see stuff like Fil-C proving it's possible, and if the performance gap can get within 10% (which seems very possible) it would be a no-brainer.
It depends how much the C software is "done" vs being updated and extended. Some legacy projects need a rewrite/rearchitecting anyway (even well-written battle-tested code may stop meeting requirements simply due to the world changing around it).
It also doesn't have to be a complete all-at-once rewrite. Plain C can easily co-exist with other languages, and you can gradually replace it by only writing new code in another language.
