I can speak for myself, with the caveat that I am a professional, sponsored player that does not tour.
I'm currently sponsored by MVP Disc Sports, and it was the brand I preferred to throw even before I was sponsored by them. I began, like most of us, as a mixed bag player using whatever discs were either given to me, acquired in player packs, or purchased because they caught my eye.
As I started throwing more, I developed a preference for MVP's plastic and began switching to their discs until eventually I was throwing only their discs. By the time I applied for sponsorship with them and began talking with some of the people at MVP, I wasn't throwing anything else.
I don't really miss a mixed bag at this point, especially since MVP has really upped their mid-range game in the past few years.
However, this is the plastic I want to throw and I'm fortunate enough that MVP supports me.
One of my friends who is also sponsored, though not by MVP, was courted by a major sponsor after a high profile amateur win. As far I know, she threw a mixed back at the time and converted her bag to that sponsor's discs.
It's a much, much different ballgame when you're a touring player, however. At that point, it's less about what plastic you prefer to throw and more about the kind of support you're going to get to stay on the road. You'll throw whatever brand is giving you the most support to continue touring, and switch to another brand if their offer of support is a significant improvement over what you currently have.
There is room for nuance here once you start looking at the various levels of sponsorship at the professional level, obviously. If you're pulling in top 10 finishes at Elite Series events routinely but are only getting the entry-level pro sponsorship, you're probably going to learn new discs for another manufacturer if renegotiation fails to improve the support you get. Also, some support is based more upon your social media presence/output than your tournament results.
And that's part of individual professional sports as a whole: brand loyalty only goes as far as both parties being happy.
I'm quite fortunate that I feel very supported for the level of play I produce while also throwing the plastic I like. It very much is a dream situation for me. My views and expectations might be different were I young enough and obligation-free enough to give up my dayjob and play disc golf for a living.