You are arguing a distinction without a difference. If socially one could not muster the will to do something then it will never happen -- it cannot be done.
We certainly could reduce the use of fossil fuels by over 90% in a few months -- global nuclear war would probably work -- but I hope nobody has the will to make that happen. Conversely we could reduce them by that much over a 30 year span, but currently lack the will. So in that case the distinction would make sense.
But few are willing to "replace this working thing for a small gain" (and you can argue about rust but when something works and the downside is a statistical risk demonstrably of low probability* ) it will be hard to find many people willing to invest in it.
Eventually these old programs will be replaced but they won't be replaced with functional equivalents, any more than, say, wired home phones were replaced with fibre optic connections. They will be replaced with something de novo instead.
* nerds like us may say the risk is significant but the level of uproar in security breaches is negligible in practice and the actual incidence of bugs due to things like use-after-free is likewise tiny.
