Most of the decisions he made over the past 25 years have been self-defeating and led directly to the decline of the influence of his own movement. It's not that "the GCC project" avoided that for ideological reason, Stallman was personally a veto on that issue for years, and his personal objection led to several people quitting the project for LLVM, with a couple saying as much directly to him.
https://gcc.gnu.org/legacy-ml/gcc/2014-01/msg00247.html
https://lists.gnu.org/archive/html/emacs-devel/2015-01/msg00...
(both threads are interesting reading in their entirety, not just those specific emails)