Wells Fargo is the biggest mortgage servicer in the United States and you don't have a choice over who services your mortgage- mine was sold to Wells Fargo without my say. I could refinance but that comes with significant fees (>$1,000), I'd lose my amazing interest rate, and there is no guarantee it won't end up back in Wells Fargo's hands again.
[1]: https://www.nerdwallet.com/blog/mortgages/bbt-mortgage-revie...
Beyond that, this question is an imponderable for me because who can say what the future brings vs. the present. I guess one can refinance with some other credit union were this to happen in the future in a manner that was not desirable for one's mortgage.
The bank that held my parents mortgage was acquired no less than four times between 1996-2012. In 2007 I got a credit card from my local credit union - right now it's being transitioned into a Bank of America credit card. The credit union still exists though, they sold their credit card division to another company which then was acquired by BoA. Funny thing is a several years after they sold their credit cards they decided to offer them again and create new ones.