NPV ain’t gonna happen any time soon - and if they do get enuf states equalling 270 Electoral Votes to sign up, the NPV might be declared unconstitutional under the Interstate Commerce Clause AND the 14th Amendment.
States have a plenary right to choose the method of selection of their electors - WITHIN state boundaries,
With NPV, the elector selection is influenced by voters who ARE NOT eligible to vote within the state.
This sets up the constitutional question posed above ...