Gecko has its own problems. Installing Gecko would fix that webkit security breach for sure, but you would end up with gecko’s security breach. So in the end it doesn’t change anything.
Actually it does, because you have options if a 0-day surfaces. Your logic only works if there happen to be multiple 0-days released at the same time on all major browsers which affect all recent versions for each browser (because on iOS, you can’t even downgrade to a previous version that could be unaffected). That will probably never happen.
Gecko has its own problems. Installing Gecko would fix that webkit security breach for sure, but you would end up with gecko’s security breach. So in the end it doesn’t change anything.
Actually it does, because you have options if a 0-day surfaces. Your logic only works if there happen to be multiple 0-days released at the same time on all major browsers which affect all recent versions for each browser (because on iOS, you can’t even downgrade to a previous version that could be unaffected). That will probably never happen.
0-days that we know of
There definitely are 0-days in every major browser engines.
As a matter of fact, Mozilla is probably working on a 0-day breach that haven’t been published by security watchdogs yet.
In the meantime, that particular WebKit breach has already been patched.
There’s no point skipping places when everything is on fire. The only thing you can do is going where it’s safer on average and stay there.