if they truly believed that Sony or Canon would improve their photography and they got the same deal from the new brand some of them would switch.
I am not sure I agree. When you are a brand ambassador you have a financial consideration. It is part of your total income as an artists. Sure they can downplay how much income comes from it but take that with a grain of salt. If they said yeah brand X pays me $50K a year and free gear or they say yeah it has a few perks but doesn't pay much which one makes them seem more trust worthy when they say this new camera is the best ever? I think some of you would be shocked at how much a photographer who has income beyond just taking pictures makes. The AP changed brands because they as an organization believed a change improved their overall needs of video and stills. If we look at sports photographers decades ago migrated to Canon because of improved performance as AF was the new thing.
If you follow sponsored sports you will see that some change either by choice or not their choice. Take a pro bass fisherman, he has a boat sponsor, rod and real sponsor, lure sponsors, electronics, motor sponsors and on and on. Do they just up and change if they believe some other brand improves their ability? Not likely, those sponsors pay the entry fees, travel cost etc. and if they "change" often they will run out of those willing to sponsor/pay them.
Take old Moose for an example. Would Canon pay him a lot to change from Nikon? Sure they would with likely a 10 year agreement. Now say Moose changed each time a camera brand came out with what would improve his photography. How likely is any brand willing to pay when you aren't loyal? They have contracts for years. The company I work for is a major sponsor in NASCAR, we have decade long contracts with the largest team in the sport. No one wants someone who just changes because the other guy is better this year.
At the end of the day if you are receiving money you are a business. That income matters. Could some of the top dogs switch brands and get paid? Sure they could and it does happen but it is less likely because they believe that brand helps them vs what the financial gain is for signing on with a new brand.
Lets face it all the camera brands can deliver good results. It is the Indian over the arrow for the most part. However some arrows fly better than others helping the Indian succeed. When you have someone that is paid by any brand it isn't as simple as they could use anything and they choose brand X. They likely choose brand X because they pay them money.
Joe Edelman is a great example. He is a good photographer going on decades. He shot Nikon since he was a photojournalist at a local paper in PA. He changed his photography to fashion, model style photography. He was not a brand ambassador for Nikon. Olympus approached him to be a Olympus Visionary and get paid. He travels around the country for Olympus and teaches classes (he was in TX and I went, I learned a lot but not what I photograph). Did Joe really leave Nikon because Olympus did things to improve his photography? Nope. He left because he is a business that has many income streams other than just taking pictures. He has a youtube channel, teaches classes on his own, paid photo jobs, paid by Olympus to teach and to promote the brand. He is one of the most savvy business people I have ever met in the photography business. The point I am trying to make is for those at the top of the game they have motivation to stay. For those trying to get to the top of the game they have motivation to want to be paid by a camera brand. Just like any pro that has sponsors, it is an additional income stream. It doesn't mean that any one brand is better, it is that is the one paying them and it gets the job done.