Was it only Quake that had such extensive modding capabilities, or was it Duke also?

In short, if they stick to the spirit of games from that time, then user content will show up and you'll both get your wish.

Games don't seem to stick around long enough for modders so much nowadays :\

Thats only half the reason for the lack of mods, the other is that the orignal game creation sdks are now free or freemium atleast so it wouldn't make sense to try to mod something if you can get supreme tools for original creation nearly for free.