Sure, they could write their own search engine (they also don't do large compute - so they'd be getting it from someone they'd be paying) but Apple would be paying for compute and licensing ... and not have any revenue from it ... and get in trouble with anti-trust or monopoly issues in Europe to boot.
Writing a search engine isn't likely worth the headaches that it would bring to Apple.
So far, Company + Map hasn't caused any problems with even bigger players and tighter interegration (Google + Google Maps and Microsoft + Nokia ( https://news.microsoft.com/2013/09/03/microsoft-to-acquire-n... )).
Just tried it and Chrome isn't supported!
Apple would not need to pay anyone to make their search (or maps or browser) the default. It would be the default on Apple devices and not used much outside of that.
Would that bother Apple? Their platform is the most valuable to advertise on. They’ll make money. The question is, will it make as much as Google was paying?