I think peak OS development was in the 70s and 80s. Every hardware vendor also had their own OS, or several. The System V OSes were just different enough to be annoying.

To name a few: UNICOS, NeXTStep, HP-UX, RSTS, ULTRIX, VMS, AIX, SunOS, Solaris, IRIX, A/UX, MacOS, ProDOS, Amiga OS.