Presumably for something as expensive as the US nuclear weapons program, they could afford to hire and fully train 10x as many people as they need to be experts in externally-obsolete software/hardware and it just have the effect of a rounding error in their overall budget?

If that were cheaper, or considered more reliable or less hackable, than upgrading the systems to run on more modern technology then why not, as long as they look ahead far enough to make sure they fund the required tutoring to get the knowledge being passed down a generation or two, and do it before getting to the point where not having enough young experts actually has become a problem.