First finding out about strange bedfellow pickups...

The idea of swapping out pickups came so much later than the invention of the pickup itself, standards would've been an absurdity for the first several decades of guitar electronics. Luckily phase really only has two directions (although two ways of reversing it so four variations), so it is usually not that big of a deal.

I remember working on some no-name '60s guitar that had two pickups, but no combined pickup setting for some indiscernible reason. The customer wanted a real three way switch installed, which we did. It turned out the two pickups were out of phase with each other, and the dang things were riveted shut and near impossible to change the phase. So in this case, two pickups made in the exact same factory were out of phase with each other because they never imagined having two on at once.

When you stop to think about it, if Fender and Gibson wanted to be pricks back in the day they probably could've lawyered up on patents and copyrights and kept 3rd party manufacturers from ever making pickups with their proprietary form factors. I have no idea if it would've held up in court, but I suspect it could've if they fought hard enough. Then, the only "humbuckers" you could get would be from Gibson, and the only Strat and Tele pickups you could get would be from Fender, unless they decided to be generous enough to license it out for a substantial fee. Other brand instruments would have had to come up with their own form factors, or at least use ones generic enough that no one would care. The third party pickup world would be more akin to what DeArmond was doing back in the day, making pickups for a variety of manufacturers to their own specs.  One of those interesting thought experiments.... Rickenbacker successfully stopped Lollar from doing toaster pickups, so it can't be that far off for what the lawyers could do.