I don't think that can be right. The Democrats have recently won both the House and the Senate. In such an election, if "winner take all" is abolished, how would they not win the presidency?
Because in states like California, Colorado, etc., vast swathes of Republicans do not bother to vote because their vote is overridden. The numbers don't work in reverse.
Just look at the county maps within blue states: these elections you speak of relied on those folks being entirely disenfranchised.
Of course it works in reverse. Plenty of Democrats are not going to bother to waste their time in California when the current electoral outcome is a foregone conclusion. Similar with Republicans in Mississippi.
If the rules changed to a popular vote where even "safe" states were up for grabs, I think there would be lots of previously uncounted "dark matter" voters who would activate and would significantly impact the outcome.
This math doesn't work in reverse because there aren't as many applicable people or relevant districts in the rest of the states.
Mississippi has far fewer total disenfranchised Democrats (in both absolute number, district count, etc.) than California has disenfranchised Republicans.
Without extreme gerrymandering, there simply aren't enough eligible-to-be-swung electoral votes to meaningfully benefit Democrats in rural states.
You do not need disenfranchisement, just apathetic voters who do not currently contribute. Right now there are ~23 million voters registered in California. 45% registered D, 25 %R, giving absolute numbers of 10 million D, and ~6 million R. Which you can handwave is 4 million Ds who know they do not need to contribute - their neighbor has their back to secure the state electoral votes.
Looking at the US as a whole, there are 44 million registered D with 37 million R. If you could round up all affiliated voters, Dems win the presidency every election if going by popular vote[0].
Just look at the county maps within blue states: these elections you speak of relied on those folks being entirely disenfranchised.