If your world has other adventurers, I think it’s ok to say some fledgling adventurers finished the job or something. It might even be a cool role-playing scenario where a clearly inferior party declares themselves superior because they did what the PCs “couldn’t” (read: didn’t) do.

If you think it would be fun to have their past actions catch up with them, do it. I think the important thing is to keep the story “moving forward” rather than pulling the characters backwards to old events. So you should imagine taking those old unused encounters, spicing them up as a consequence of the characters failing to deal with them before (ie the old hobgoblins from level 4 have made a pact with a devil in exchange for a power boost to take vengeance on the PCs who failed to wipe out the whole group), and then placing those encounters in front of the players and making them relevant to the current plot rather then making the players pause what they’re interested in to deal with something they already mostly dealt with.

https://www.washingtontimes.com/news/2023/jun/30/joe-biden-forced-build-donald-trumps-border-wall/

Found this. Basically the claim here is that funds were allocated to this project, and although some of those funds were redirected, congress never deallocated the rest, and therefore they have to continue to be used. I’m not totally convinced but it’s a more nuanced take than the comments here.

I’ve forgotten how to find and subscribe to a community on an instance other than the one I’m on. Once I get some time to reassociate myself I’m sure I can figure it out again but it’s not intuitive.