If it’s no longer relevant, it should be possible to convince small states both red & blue to accept an obviously superior system.

But it is still relevant for just about all the reasons it was relevant then.

I think if you had some way of forcing the issue, and telling states like Kansas and Mississippi that they didn't get to have disproportional control over the federal government anymore, and if they didn't like it, they could just stop being subsidized by California and New York, either they would go along with it, or they would secede from the union and deteriorate into third world countries.

Why would states persist in a union that made them irrelevant? The politics are still as relevant today as it was then.

It’s way harder to leave than it was to refrain from joining.

Another major factor is that Americans mostly see themselves as citizens of the USA first, and citizens of their state a distant second.

Because the alternative is worse.