There's a very, very simple rule in business. Everyone is expendable. Workers, supervisors, managers, owners. Leaving your long term job and starting over might not be a bad thing for them. They can hire younger employees and pay them peanuts. Leaving for yourself may in turn be beneficial as well. Nothing is worse than a toxic boss.
It's your neck. You can do whatever you'd like but the mentality that you can't be replaced is inaccurate. Upper management will rather everyone leave than looking like they made a mistake. When you're gone and they have fired your old boss, chances are they're not going to offer you your old job back.
I'm set in my position. I get along very well with my coworkers, boss and patent company. I also receive many perks that others do not. I'd be lying to myself that I couldn't be replaced.