Nothing is being taken away from Mike_D, we are just closing the loophole where he had a house. We're not targeting Mike D the specifically, just the behavior of living at his address.
/s
More generally, these aren't loopholes. They're often clear and explicit laws that specify exactly who has the right to water. They are often explicit contracts signed by the government. Some are as explicit as you have purchased the right to use 100% of the water from this River for all of time, or you are guaranteed x amount of water at y cost for a hundred years.
Sellers remorse is not the same as a loophole.
At some point in the history of Mike D's hypothetical property, the government acknowledge the private land grant. The law has written and interpreted acknowledges the transfer of property from owner to owner overtime. Changing the law to unwind property rights is an example of ex-post recontracting.
The same is true with water rights codified and law and case law