Here's a little hindsight thought experiment: if
the SAGE hardware had been
 reimplemented in solid-state in, say, the late 60s, would it have been more
 cost-effective than running and maintaining the tube version for the
 remaining 15 or so years of the system's life? 
"Cost effective" would never have been an issue. Running an AN/FSQ-7,
as expensive as it sounds, is peanuts compared to just keeping one
F-102 aircraft fed and maintained.
--
Will