Instead, they left it up to the individual states to decide how to select electors, and initially most of them opted to have the state legislature choose the electors, not a popular vote of the state's citizens. It took decades for all states to converge on having a state-wide popular vote decide electors, and even today, "faithless" (rogue) electors -- who vote against the popular will -- are not punished in a number of states!
Given that the Constitution explicitly gives power to the individual states to decide how to select electors, there's nothing inherently "changing the formula" in a state deciding to select electors based on the entire country's popular vote tally, instead of just the state's vote tally.
Rather, the main argument against this is whether it violates the Compact Clause in article 1 by virtue of being a compact between states.