If Google stopped supporting Firefox today, Bing would still pay to be the default engine. If bing does not pay, Yandex would do.
My point here is Firefox still has 2.71% market share, a lot of search engines operators would pay Firefox good money to be their default engine.
Their revenue stream is guaranteed as long as they have good amount of users.
But they actively choose to ruin it.
Because Google is already paying.
If anyone else offered to pay what Google pays Mozilla would drop Google like an hot potato (like they did Yahoo).
So I wouldn’t count on any mythical alternative unless Microsoft decides to waste more money promoting Bing (which wouldn’t really be an upgrade privacy wise).