That would hurt Apple as much as Google, and maybe more.
Google pays Apple more than $1B a year to make it the default, as well as a share of ads served on iOS, estimated at being up to another billion. Looking it up, Google made about $9B on iOS ads, but only about $4-5B was from mobile Safari (the rest is from in-app ads, etc).
Even if the default was not Google, many users would just change it back to Google (I know I would - Yahoo and Bing suck ass), so say Google would lose $2-3B in revenue but then save $2B in payments to Apple. So it's a good deal for Google, but not a great deal. And, it's a also a good deal for Apple, because unless can find another ad provider who will pay more and generate more ad revenue, Apple would lose some amount that $2B a year they get from Google.
So, they are both making a reasonable return on the deal, and would both lose out if they cancelled it. Which makes sense, they are both very well run companies who don't throw around money without understanding their return on investment.