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.
I'm away from my PC/consoles for the next year but I've been all over Polytopia on mobile. Easy to learn hard to master. Very complex little game that I can play through a few times a day with lots of replayability.