The very existence of Bing is the counter argument to your point.
Bing has a roughly 8% desktop market share. PC has a 70% market share.
That means about 11.5% of windows computer didn’t switch away from Bing.
Assume a similar share on apple devices. Apple has two billion active devices.
Assume that is 1.5 billion people, round down market share to 10%: 150 million of the richest customers on earth. What is that worth to google?
Not to mention another competitor could buy the spots for less. If google leaves an opening someone would try to compete. Even Apple might.
So there’s future downside risk along with the risk to present profits.