So the descendents of someone who secured a deal based on a completely different socioeconomic context and then died over 100 years ago should be given priority over those who live today and those who live tomorrow? Sounds like theft to me.
And if you are going to go "property is theft", well, you're going to have to justify that with more than just the slogan itself, because it's a huge change, and the bigger the change the more justification you need for it. And you're going to have a hard sell, because people like owning stuff.
But also, federal/state/municipal authorities do eminent seizure on private property all the time, when it is necessary for a larger public need. Forcing title holders to sell back their "water rights" and then putting them on level footing isn't that much different, is it?