GPS was always a civilian system
President Reagan authorized the development of GPS - a civilian navigation system for civilian airliners - in 1983, after the Soviet Union shot down an airliner. The first satellite was launched 6 years later in 1989, and the constellation was complete in 1994
It didn't take 10 years just to notice that civilians were permitted to use an existing military system: it took 10 years to develop a new civilian navigation system suitable for airliners.
Electronics and Communications weren't at some kind of dead stop during the 1980's: like computers and networking, satellite technology completely changed in the 1980's. The GPS satellite launched in 1983 was not 1970's military technology: it was new technology, developed for civilian airliners in response to the loss of a civilian airliner that went off course and was shot down.
The military was able to piggyback on the back of the new civilian navigation technology, as they always have: the Allies collected civilian maps and photographs of Europe prior to D-Day landings in Europe, and removed road signs in Britain for the same reason. And the development did happen inside the "Military Industrial Complex": this is the well known method of pork-barreling and industry protection in the USA, but that's much to the disgust of the military, which would like to appropriate all of the 'military' budget for military purposes, rather than having it used for things like developing a civilian navigation system for civilian airliners.