Google overpays quite a bit so they have a viable competitor to point to for chrome. If the payment tracked FF's usage numbers it would be way lower now. It makes no financial sense for any other search engine to pay that much.
That's assuming they could even afford it. Most can't
The correct solution would have been for Mozilla to pursue alternative income a long time ago. Owning a browser gives you a lot of leverage. Instead they made a half-hearted attempt a few years back and half the products failed. I don't know why FF fans were so comfortable holding them as the savior of the web when they were entirely funded by Google.
And now... well I don't see a way forward either. Maybe it should just die then.
There are bots too but you can tell a bot. Usually anyway.
They didn't all leave. It's not a hive mind. Especially since reddit got popular and the original base got fragmented and whatever.