Sometimes it takes quite a while for a theory to be practically implemented.

Work on Scramjets started in the 50s, with engines working in laboratory conditions in the 60s, but didn't operate in real flight conditions until the 90s and have only recently started to approach practical use - it's hard to say exactly where they are today since most such projects are classified.

I'm not sure when the theory for FFSC engines dates to, but the first example was built in the 60s. However it was unable to sustain stable combustion and the first stable engine wasn't tested until the early 2000s. The first test flight of an FFSC engine wasn't until 2019, and the first practical use will probably occur this year.

The basic theory for fusion dates back to the 1920s, with proposals for fusion power specifically dating to the 1950s, but it still hasn't gone anywhere, yet. We have been making steady progress, so it may still go somewhere given more time. The recent scientific breakeven at the NIF was a significant, if not directly applicable milestone.

Advances in computer control technology have been instrumental to a lot of the recent progress in the aforementioned applications. Having the theory is one thing, being able to control a complex and delicate process in practice is another.

Maintaining continuous rotating detonation has proven quite challenging in the past, typically breaking down due to instabilities in a matter of milliseconds. The fact that NASA were able to run this engine for what looks like about 8 seconds is very promising indeed.