And I never came across a photojournalist who used an OM.
I did.
Jane Bown did.
The association of black with professional quality probably arose because more of the pro oriented bodies came out in black. Many of the advanced amateur cameras did not, or were rarely seen in black.
The OM bodies were the heart of the most extensive photo-micrography system available. They also offered extensive capabilities to those who needed bulk backs, data backs, and a bunch of other niches.
"System" cameras were cameras that offered a lot of flexibility. Many of the systems had particular strengths. Photo-journalism was important, but only a tiny percentage of cameras were used by photo-journalists.
I sold a lot of cameras over a number of years. I never sold a special purpose interchangeable finder for a 35mm camera, never had one in stock (and we had lots of accessories in stock - at least one store catered to university purchases) and never had a customer enquire about one.
I did sell a few clip on angle finders - they served the purpose for just about everyone.
The 35mm cameras that offered interchangeable were often great cameras, but very, very, very few customers were interested in the finders themselves.
And as for the cost issue with shutter priority, the real cost was in having to change the lenses to work with it. The FD lenses were designed from the beginning to offer it, so Canon didn't have to incur that cost for the AE-1.
FWIW, even though I sold lots of A-1s, they never appealed to me.