"The real story here is that we typically see one or two kernel-level LPE (Linux privilege escalations) vulnerabilities that affect multiple distros/versions per year"
LPE: local privilege escalations .. Not "Linux priv escalations" ..
What do they do ? Give power to their management ? "I am responsible for 50 people, I am important". "I managed over 250, I am important, give me money".
Are you not confusing GC (freeing memory) with the memory allocator ?
Memory allocator: tcmalloc, jemalloc, they are concerned with fetching (and releasing) pages of memory from the OS and allocating objects for the program
GC is only responsible for saying to the memory allocator "this object is no longer used"
But concurrent access, so synchronization is required (lock or whatever), so overhead :)