Your argument goes through just as well for the base language itself. What's the point of having a C standard at all? Since you could just port your compiler to any platform...
The point of a language standard is to have multiple compilers and to be able to write code to some standard. The difference between these points is that with a "stdlib" alternative library, you can still have multiple compilers and write library function calls without following the standard, but simply the library's API. The reason I believe the stdlib shouldn't be a standard is for the same reason that libpng, libjpg, libzip, etc. aren't part of the C standard. It should be the software vendor's choice of which standard library to use.
