The brand wars are mostly an amateur thing.. once you decide on a brand if you're really serious and especially if you're running a business you don't jump brands over little differences in sensor performance because it's too much financial churn and you lose your muscle memory switching to a camera with a different control layout.
But it really sucks if your gear goes down and it's hard to get it serviced.
Canon's service is fast, excellent, and much more affordable than replacement.
This is a -huge- thing for some folks.
Have a colleague who does some light pro photography (i.e. weddings, concerts, etc.) I wouldn't call him a Canon zealot or anything, although his first comment at my Sony was 'Well, it's not a Nikon so I can still be your friend.'
We were showing each other shots and he was amazed at what I was able to produce with an e-mount. So he decided to give it a little spin.
All the feedback was around 2 points:
- It took good shots, but.
- Everything was so unfamiliar to him that he missed shots due to not being able to find anything.
Some time later I ran into him again; he had tried Sony cameras a few more times but just couldn't get comfortable enough with the layout.
(The Sony layout -is- kinda bad, even if it's what you've learned on.)
The Sony layout on the otherhand is just terrible. Sure, you can customize all the programmable buttons and menus and it gets much better, but it still takes a long time to learn. Also, had to watch a ton of youtube videos of people changing settings on their camera (the fact that there are so many for Sony cameras should hint to how terrible the menu is).
For instance, zoom rings on Canon and Nikon lenses go opposite directions; exposure compensation dials go opposite directions etc.
And when you're talking about camera systems, the camera itself (which is where Sony currently has the lead in mirrorless) is not the biggest factor in choosing a brand.
Canon and Nikon both have tons of specialist lenses that are still not available (or only have one flavor specializing in one space, at one very high price point) on newer mirrorless platforms.
And if you've invested $50k-100k in lenses (e.g. for a photography department) are you going to turn on a dime and dump that gear for new fancy Sony equipment that costs more than your old glass? Likely not.
And many PJs still need some of the qualities that you can really only get in top-end Canon or Nikon DSLRs (as someone else said, ability to drive nails with the camera, and huge grip).
As a concrete example, imagine if you were tasked with taking footage or stills of one of the protests at night. Would you rather bring a rock-hard large-gripped body that could even be used for protection? Or a smaller body with a screen that would break from a minor scuffle and cheaper plastic, less waterproof construction?
As long as the flange distance of the sensor from which you're adapting is significantly more than that of your camera and you manage to decode the autofocus communications protocol, you can adapt them.
Short version: optical physics, proprietary link for autofocus, capability differences
Really aside from Nikon, most AF protocols are fairly well understood and can be adapted to other lenses.
(in the case of Nikon, sometimes they go out of their way to break others using their systems/protocols without a license...)
But you can use larger mounts on a smaller mount (with care!) May not need a bigger tube, but perhaps a longer or shorter one.
However, take a look at articles like this[0] to see just how prevalent Canon gear is at professional events. Apparently, they even have relations with a lot of press outfits to have them exclusively shoot with their equipment.
[0]: https://www.shutterbug.com/content/battle-canon-vs-nikon-sup...