Eh, forks can legally appear, because it's free software. But maintaining them? Improving the product security, let alone adding new features? That's hella expensive. Hobbyists aren't going to write a competitive a web browser.
So, it's important to have a strong Mozilla with a good revenue stream.
Yes, I don't see either small teams keeping up with the amount of features we currently have (and security is a direct factor of the amount of features), but I would rather see them scaling down browsers than making sure we can keep adding to them.
Also, beware to not discard "hobbyists" too easily. Hobbyists did create GNU (and still are maintaining it) and early linux. And Blink comes from Webkit, which comes from khtml, made by hobbyists.
There’s no way they could revert to hobbyist projects and survive. Either they would die off or the money would fork them.