Apple could make their search engine subscription-based, or roll it into Apple One.
It could also be a hedge in case Google ever stopped paying Apple, either due to regulatory concerns or because Google feels they don’t need to go through Apple anymore.
You don’t always wait till things turn bad before scrambling for a solution. Sometimes, you develop a trump card first, then hide it in your sleeve for the right time to play.
I can see the merit of Apple developing one, or at least be seen to be developing one.