If the revenue is worth the cost, why leave it to chance?

Internet Explorer seemed impossible to beat when it was packed in with Windows, and yet Chrome is now the dominant browser.

I'm sure that isn't lost on Google. The change might not happen overnight but why even let it start if you can afford not to.

The payoff disincentivizes Apple from competing and ensures Google does not need to compete. I would assume there's also some kind of exclusivity such that Apple is also incentivized to promote Google search over others.

Google would likely win the melee but not without unknown effort. Loss of users + risk + higher maintenance cost is all part of the equation.