Richard wrote:
  Lots of stuff from the 70s and 80s survived, otherwise
we wouldn't
 have any PDP-11s, VT100s, etc. 
Only a tiny fraction of the PDP-11s, VT100s, etc. survived.  The vast
majority were scrapped.
While there may have been a lot of ISC terminals made, for some value of
"lot", the number obviously was orders of magnitude fewer than VT100s.