ps -- I'd talk to a lawyer (it's free for a consult! Write down the details so you don't miss anything and can be dispassionate, then ask one or two to review your situation and options!) about filing a wage claim for that $60k, or some other type of compensation.
One last thing. You know what you call people whose leaving would crater a company? Owners. If you're that crucial and only have < 5%, you're a sucker. Sorry man. This is a lesson; don't let it happen again.