Recompiling existing software written in C using Fil-C isn't also a great idea, since some modifications are likely needed, at least for fixing bugs found with usage of Fil-C. And after these bugs are fixed, why continue using Fil-C?
This would ultimately save much of the overhead associated with tracing GC itself.
Yes, this means that C is one of the slower languages around. That’s fine; computers are fast. If you want to write high-performance code, there are plenty of faster languages to do it with.
"A consequence of this principle is that every occurrence of every subscript of every subscripted variable was on every occasion checked at run time against both the upper and the lower declared bounds of the array. Many years later we asked our customers whether they wished us to provide an option to switch off these checks in the interests of efficiency on production runs. Unanimously, they urged us not to--they already knew how frequently subscript errors occur on production runs where failure to detect them could be disastrous. I note with fear and horror that even in 1980 language designers and users have not learned this lesson. In any respectable branch of engineering, failure to observe such elementary precautions would have long been against the law."
-- C.A.R Hoare's "The 1980 ACM Turing Award Lecture"
Guess what he means by "1980 language designers and users have not learned this lesson".
Because Fil-C might be a good way to debug future code?
Your question reads like, "Why use a debugger?"
That’s why even and especially if a C program runs in Fil-C with zero changes, you should use the Fil-C version in any context where security matters