Ironically, I find it hard to believe that the majority of iPhone owners would know how to use anything else. Today's America has been preconditioned to "Google it" and "look it up on YouTube" for years. If all roads lead to Rome, cutting off the Spotlight search default might not add up to much in the long run.
If that's true, why is google willing to pay so much to be the default?
One reason is that Apple specifically blocks their users from selecting a custom Spotlight search provider on iOS. Google is probably willing to pay whatever it takes to lock themselves in as the abusive and immutable default provider.
It's ironic that you framed Google as the abusive provider there.
I consider them both to be exploiting the status-quo. Apple and Google need to be punished for creating systems that exclusively benefit themselves at the direct expense of the user. It's irresponsible, anticompetitive, and the sort of business relationship that necessitates government intervention.