> Can't they just take Google's yearly $600,000,000 payment
They can do that for as long as Google is willing to pay. Without additional revenue stream, the day Google decides to cut cost and stop sponsoring Mozilla, that's the day Firefox will run into big trouble. Any additional revenue stream is going to help.
It's not a sponsorship, it's a sale.It's also made such deals with Yandex, Baidu, and Yahoo before. The sale is for eyeballs on your search (ad) service. Firefox is getting in a precarious situation with it though as the number of users on Firefox has been decreasing (in absolute terms) despite the number of web users increasing.
It's also a part of their anti-trust defense strategy. Firefox staying alive as a marginal but existent alternative to chromium based browsers is currently good for google.
They can do that for as long as Google is willing to pay. Without additional revenue stream, the day Google decides to cut cost and stop sponsoring Mozilla, that's the day Firefox will run into big trouble. Any additional revenue stream is going to help.
I am no CEO but that seems very clear to me.