A look at one of the Soviet Union's first SLR cameras, the KMZ Start
dpreview.com
dpreview.com
The Nikon F is an early 35mm SLR with a pentaprism. Professional SLR’s go back to the 19th century. For example Graflex. http://camera-wiki.org/wiki/Graflex_reflex_models
Here is Dorothea Lange with a Graflex SLR. https://www.shorpy.com/Dorothea-Lange?size=_original#caption It may even be the camera she used for Migrant Mother.
Prior to the Nikon F there were 35mm SLR cameras with other types of finders, e.g. waist level.
Hasselblad produced its first consumer SLR’s in the late 1940’s and the iconic 500 series was launched two years prior to the Nikon F (1957 v 1959). https://www.hasselblad.com/inspiration/history/first-consume...
The Nikon F was the first SLR that was so rugged and had such good lenses that it really made professional press photographers switch from (mostly Leica) rangefinders.
As you say there were plenty of SLRs before but they were more seen as fancy cameras for consumers (also there were all kinds of limitations like the mirror not automatically returning for instance) and the optics were usually inferior to rangefinder lenses due to the need to account for the extra space for the mirror box. The viewfinders were also often quite dim.
(The F system did follow the pattern of most contemporaries to design stuff so that it would do what is needed now, with little regard for future developments. That's why Nikon introduced Ai (and then very shortly after that, Ai-s) mount variations, or why Canon had like five variations of the FD mount; almost all companies operated like that, with very few exceptions such as Konica AR)
Which one did the professionals end up using? Go on and search the web for some photos of war photographers in Vietnam and see what is hanging around their necks. Spoiler, usually one or two Leica Ms and a Nikon F for the times when you need to keep your distance.
The Nikon SLRs picked up a reputation after being used by professional photographers to cover the Vietnam war years after the release of the first Nikon F. Were they professional cameras before professionals started using them?
I have failed to mention the Asahi-Pentax, released in 1957, which had everything the Nikon had two years later. Why is the Nikon F the first professional 35mm camera? What made it special enough to obscure years of camera development?
There was a war on in the teens.