It's a bit of a chicken and egg situation - because in the past Canon and Nikon have dominated, lots of pro's use Canon and Nikon, so there is a large support service and they are widely stocked.Not arguing but adding detail.
There's a reason most pros shoot Canon or Nikon. Pro level support, any specialist lens or piece of gear that might take your fancy etc.
I do recommend other cameras for specific needs, but there is a good reason for that lineup of hundreds of white lenses at major sporting events, it's not a coincidence.
Whilst enthusiasts are poring over dxo scores and slagging off Canon for their crap sensors, the pros are out there queueing for the next Canon pro body or lens (are they daft?).
I know that the others are outperforming in some areas and catching up in others, but the fact remains there's still clear water between how CaNikon service pros and everyone else.
Other brands have smaller market share, so have smaller support service and are less widely stocked.
Sony are building up their pro support service, but it's still a small operation by comparison (I don't know about other brands support options).
It's certainly a point to consider, but it's unclear if the OP is looking to invest in the level of kit that leads to pro support services (not sure on the requirements for Canon / Nikon to enter such support programs, but assume there are some requirements or costs involved)
Why are pro's queueing up for the next Canon / Nikon body?
Certainly not because they are daft (some might be, but in general not) - but for many it is because they either have a significant investment in existing gear (both in cash, and in time becoming familiar with it).
There is a cost to switching brand - and getting the next new model in their existing brand is probably cheaper than switching.