The F-14 dev actually started in late '66, and the F-14 didn't enter IOC until 74, so 8 years. F-15 had a 9 year development process, as did the F-16. F-22 took 24 years. The F-14 was also very rushed, and a lot of it's capabilities languished until new engines replace the TF-30 (which it inherited from the TFX project).
I'm not saying that is has to take 20 years for a new fighter, I'm saying it will. The aerospace community has consolidated incredibly since Grumman was cranking out the Tomcat. You have basically two US firms capable of building fighters now, compared to the 7-8 that existed in 1970. Add in the screwed up Pentagon procurement process, and 20 years looks optimistic.
It's not really fair to compare SpaceX since they don't have the Pentagon and Congress breathing down their necks to create jobs in 50 states.
Of course there was prior work, going back to 1903 at least, if not much further. The point is that the lag between the start of official F-14 procurement and planes flying was not terribly long. Just as today you don't have to start with a clean sheet design, there are already systems, engines, guidelines, missiles, etc. that you can borrow from other designs (including the F-35) to produce something better. The only reason F-35 isn't cancellable currently is political, not technical.
What would you have the AF/Navy/Marines fly instead? The Harrier is so far gone we've had to buy the entire remaining fleet from the Brits. The original Hornet the Marines use is on its last legs, and the Super Hornet is nowhere near good enough for near peer competitors.
F15/F16 can soldier on for a decade or more as long as they're out of range of S-300 and above SAMs. There aren't enough F-22s in our inventory, and no prospects of re-opening that production line.
The AF and USN are investigating a 6th-gen fighter, but that will be a long ways off since money is being spent like crazy on the B-21.
And back to the development process; modern aircraft are more complex, and do take longer. You can't just go back to a design/devel project from the '60s anymore than game developers can crank out a AAA game like Dave Theurer Atari did with Tempest.
And if you look at aircraft projects around the world, they all take forever. Look at Rafale, or Eurofighter, or Su-57. Developing aircraft has to include political considerations, (look at the TFX proj) as much as technical.
I'm not saying that is has to take 20 years for a new fighter, I'm saying it will. The aerospace community has consolidated incredibly since Grumman was cranking out the Tomcat. You have basically two US firms capable of building fighters now, compared to the 7-8 that existed in 1970. Add in the screwed up Pentagon procurement process, and 20 years looks optimistic.
It's not really fair to compare SpaceX since they don't have the Pentagon and Congress breathing down their necks to create jobs in 50 states.