Even when I was a wedding photographer in the 1980’s, I carried my medium format Mamiya, and a Canon 35 mm. And I carried three sets of flash cords for the formals in the church. I remember more than one where somewhere along the line the first 2 stopped working. Of course when I went home, they all worked flawlessly.

The primes we had in the 1990s are rather inferior to the consumer-quality zooms we have today. You used primes from(my time) 1973 to the early 20teens because zooms were so bad. And of course, there was no such thing as autofocus, and the concept of auto-eye focus before powerful computer chips was hard to imagine.

when I started in professional photography, the average press, wedding, or industrial photographer used a 4x5 "press" camera. One camera, a strobe, and a box full of film holder was enough to lug around. The good thing was, you learned to make the best of that simple gear and workaround many issues. That same concept followed in medium-format usage. Once 35mm became, more or less the STANDARD press photographers'' and photojournalists' tools, multiple camera usages came into vogue- even among folks who did not need so many.

In the days before zoom, lenses were as good as they are today, for photojournalists, juggling lenses with one body was impractical. Waht do you do with a 300+ prime, where do you keep it? Open bodies, lens caps, all things to fumble with in fast shooting situations. What do you do if a single body quits in the middle of an assignment?

Nowadays, especially for pros, you are playing Russian Roulette if you only carry one body. There are too many electronic circuits and onboard gadgets that can go south.