Advertisers can’t. What are the other choices - Bing, maybe Facebook?
Furthermore, search users aren’t even Google’s customers. Advertisers are. So whether users can switch is not an interesting question.
There is a separate suit [2] that is specifically about advertisers, not users. It was brought last year.
You assert that only the second one is an interesting question. I'd say they both are. Contrary to your statement, search users are absolutely Google's customers. But a company can have more than one set of customers.
[1] https://en.wikipedia.org/wiki/United_States_v._Google_LLC_(2...
[2] https://en.wikipedia.org/wiki/United_States_v._Google_LLC_(2...