Apple already is a company that sells some ads, if that’s what you mean by an “ad selling company.” If you mean, a company that primarily sells ads as their main revenue stream — that would be bad.

I do not see how one follows the other.

Search does not require advertising.

Services do require monetization, or you have to subsidize them through some other revenue stream, yeah?

Advertising is probably the simplest way to monetize search. Certainly other strategies could be tried, but that one at least is well understood, and nigh-ubiquitous.

Apple monetizes their services by either tying them to expensive hardware or charging a subscription fee.

For Apple it could just be a customer acquisition cost.

Google pays Apple a lot of money to be their search engine. Presumably, if they became a competitor, Google would stop the $15 billion payments and Apple might want to make that up.

The loss of that payment would be part of the business model, but companies really don’t think in terms of making list revenue up.

If there is an opportunity to increase revenue (subject to mission/values/etc), you take it. A decrease in another line of business doesn’t matter. Or an increase; you wouldn’t say “hey we’ve got this amazing ad opportunity but the AirPods business is killing it so let’s not bother.”

I guess the nuance here is that the business decision might be “our search, with our ads” versus “Google’s search, with their ads, plus $15B/year”. In that case I think there’s a reasonable argument that users are better off with Apple’s ads than Google’s.

Yes, by acquiring new customers who then buy further products and services.

It could also just be sold as a service, or an addition to iCloud+.

It doesn't require it, but it provides you with Google's one big magic trick:

If someone searches for "hotels new york" you have what's called a motivated customer. He or she is much more likely to click on an ad for New York hotels.

It's one weird trick, but man, is it a good trick.