[Galadriel55]

I think the point isn't that organized resistance in key centres would be holding Sauron's might back. At the Council of Elrond they predict that if Sauron gets the Ring, sooner or later even the most strongly defended strongholds would fall, and eventually he would have all official control over Western Middle-earth. But unless he butchers every single person - and that is NOT his goal; whatever the end-game is, it's about control, not anihilation, - he cannot wipe out all forms of resistance. In the words of Beregond, "Hope and memory shall live still in some hidden valley where the grass is green". Even if the fortresses fall and there is no major defense point left, people will find a way to carry on the message. Of course any overt rebellion must be put down immediately, but with generations of patience slow work may pay off.

Historically, it is possible to force people to assimilate into a new rule sufficiently to make them forget the messages of their past generations. However, I feel like Gondor has such a deep-rooted history that it would be hard to unroot completely, even with severe prohibitions, deportations, and other measures. Would Sauron really have to slaughter all the Numenorian descendants to keep order in his new world? Would he foresee the need for such a step before it's too late, before small groups are scattered far and wide, requiring significantly more effort to find and eliminate?