I don't know about TWENTY YEARS, but I for one would probably be willing to dedicate a good 5 years at least to this game if I were suddenly thrust into the helm.
Mainly because I think that's all it would take. I wouldn't give two hoots about 90% of the stuff in adventure mode, and would treat it only as a convenient sideboard tool for manipulating stuff between (in time and space) fortresses. Such as ferrying things around, or seeding characters to be a member of your fort. Thus, stuff like prison terms and reputations and dialogue and blah blah = totally unnecessary.
And fortress mode is significantly further along already. The arcs there are far more "doable" and IMO less controversial or things that most people would have a difference of opinion on, and could be themselves finished more rapidly.
Also, much of the charm of DF is after all hilarious random unpredictable destruction and violent capriciousness, which by its very nature is at odds with adventurer mode. Either you embrace that, and 99% of the time your adventurer is a bloody smear on the road within half an hour (frustrating / not that fun), or you fix it, and lose all the DF spirit, rendering you with just another not so exciting roguelike. Also not that fun, might as well go play Skyrim. Seems lose-lose to me. Fortress is where it's at.