I think you are generally correct but the two examples you gave "triggered" me ;-)

What damaged would there be if gcc or LLVM did decide to not support x87 anymore. It is not much different from dropping an ISA like IA64. You can still use the older compilers if you need to.

Similarly, what is varargs used for? Pretty much only for C and its unfortunate printf, scanf stdlib calls. If a backend decides not support C, all this headache goes away. The problem is, of course, that the first thing every new backend designer does is to write a C frontend.