I know that some photographers enjoy speed and convenience. But aside from "horsepower under the hood" I think that clients may become the arbitrator for what's a professional camera.
If they can get photos that they think are beautiful ... and certainly can be beautiful ... I think that's what will matter. They may not have a clue or care if the image came from a T6i, Canon 5D mk iv, or Sony a9. But if it looks nice, that will probably set the bar.
You can designate "pro" to be equipment used to generate income, but I see it a bit differently. I see the "pro" designation, as it concerns equipment, as equipment designed to be used consistently, while maintaining a high level of performance and reliability. Think of it this way, you can walk into a Harbor Freight and buy all the tools you need to renovate your home or restore a car for dirt cheap, but those won't be the tools you see the pros using because they aren't reliable. They simply don't last, or they don't do the job well enough, or are frustrating to have to work with (often all three).
When it comes to a "pro" camera, it should be easy to use (ergonomics), durable (build quality), and produce high quality images (pretty much every ILC out right now). It should also perform well in terms of responsiveness and data throughput, and be able to function in a variety of professional situations, from fast paced event shooting with on camera flash to tethered studio work with a PC Sync connection for strobes.
Disregard megapickles, resolution, mirrorless vs. DSLR, and simply look at build, ergos, handling/responsiveness and studio compatibility and you'll see a certain stratification of bodies that remains despite the outdated nature of sensors. Of course, as we all know, it still doesn't matter in the end. The quality of the tools matter to the artists, not the buyers. There are kids out there turning out work with their phones that makes me want to sell all of my kit and take up stamp collecting. 



