The Mig-21/Mirage-III/F-5 upgrades have been popular for one reason- Cost. Such aircraft are not a threat to their vendors hawking newer platforms. The 21 or Mirage-3 don’t come anywhere close to anything to a Mig-29 or Mirage-2000.
That’s not the same case with a souped up F-16 like the F16U. Barring all aspects stealth, it can match or even better the F-35 in some parameters. Which is precisely the reason why LM happily peddles barebones upgrades like the F-16V variant.
Otoh, something could come along and change everything, maybe drone, sensor or weapon tech, that could be as much of a game changer as jet engines were to pistons. Then they're all scrap for the boneyard and smelters.