Hm. To me, it seems that there's a problem with a bare majority of the states deciding to completely change the way we elect a President, especially if the other states have opposed this compact. I really don't want half the states telling the other half of the states that they don't have a say anymore, especially if they have rejected this compact.
They don't have a say right now. The system is that every state legislature can decide in whatever manner they desire how to choose their Presidential electors. No state can change this.
Having a popular vote plurality election to allocate all of a state's electoral votes is a perfectly legal way of doing things should a state choose to do so. It's also 100% against the way the founding fathers intended this rule. Remember that. We're already exploiting a loophole to make things democratic. This just takes it one step further.