Well he and others moved away from UNIX, created Plan 9 and Inferno.

In these efforts they also tried to create successor languages to C, Alef and Limbo (for userspace), and never cared that much about anything past C89.

He has settled with Linux because that is the winner of UNIX wars, which predated Linux wars on Distrowatch.

As for the original UNIX,

"Using Unix is the computing equivalent of listening only to music by David Cassidy"

-- Rob Pike