As I have been saying for years, why spend billions on developing an entirely new airframe and weapons platform when you can simply modify an existing one much cheaper. Stealth has to do with shape and materials more than anything and changing the shape is easy. Now add thrust vectoring exhaust, forward canards, exhaust heat shielding and you suddenly have a 40 year old airframe that is more deadly than ever.
There are refinements in propulsion, materials, weapons, etc. but the laws of aerodynamics don't change much.
I'm not a pilot but I recall someone telling me that with updated weapons, electronics, and engines, the F-15 and F-4 would be closely matched...