I don't know. If your role is that important, they'll make you an offer to make you stay, which, unless you really, really, really want to leave, you'll take, because it'll be very good. Then they either work on making you not the single point of failure, or they'll accept you being that important and just go on paying you appropriately for it.

I see "wanting to leave" as either absolute ("I hate this company, no amount of money can make me stay") or relative ("I could make more money / have less work / do more interesting stuff elsewhere"). The latter can totally be shifted by adding significantly more money, at least for most people.