So if someone had a pickup problem from say 1954-1969, they just sent the pickup back to Gibson/Fender/Rickenbacker/Dearnond/Epiphone/Danelectro/whoever to be rewound by them in house?
Back in those days IIR these were all separate companies, kind of like how Pontiac and Oldsmobile were once separate from GM in the early 20th century.
Also I would say Bill Lawrence was one of the first, although I'd say DiMarzio was the first to do it on the scale that he did.
That said, despite his head start, I think DiMarzio is most behind now. Since the 1990s I'm guessing EMG and Seymour Duncan dwarf DiMarzio in sales.
Why? Look how many guitars come stock with EMGs/Seymour Duncans. It's mostly Ibanez guitars that come stock with DiMarzios.
Plus, a wider variety of players play EMG and Seymour Duncan pickups. DiMarzio tends to be endorsed by mostly shredders and virtuosos of the Shrapnel Records variety.
This doesn't necessarily matter, but I can more or less name a dozen or so DiMarzio artists. EMG tends to get pigeonholed into metal, yes, but how many small to mid tier metal bands are there using EMGs?
Meanwhile, Seymour Duncan's passive pickups are used by pretty much everybody across all genres.
The Seymour Duncan story is amazing in that they went from being an aftermarket pickup to being the stock pickup of choice for many passive players. SD largely displaced Gibson and Fender at their own game over 40+ years. Very impressive.