Here's one: even just switching from gcc or msvc to clang, in projects that really want to, takes years.

Here's another one: the Fil-C compiler is really young, so it almost certainly still has bugs. Those compilers that folks actually use in anger tend to get qualified on ~billions of lines of code before anyone other than the compiler devs touches them. The Fil-C compiler is too young to have that level of qualification.

So "immediately everywhere" isn't going to happen. At best it'll be "over a period of time and incrementally".