"The only way to do serious computing - servers, supercomputers, science?"
Disproof by counter-example:
Servers: The last few decades, Microsoft's proprietary OS and commercial proprietary applications/RDBMS from MS, Oracle, etc. dominated the server landscape;
Science: Python's become the most popular platform for science research. Its most popular implementation CPython, is licenced in the BSD style. So open source, but not RMS's vision of pure "free software".
There was free software before GNU, and there'll be free software if GNU or FSF ceases to be, or be led by RMS. His contribution, valuable though it was, was made a long time ago.