Even in a smaller corporation I'd be very careful. I try to put myself in the place of the boss there and to have some sysadmin walk up to me and say hey Jacques, I'm leaving today, here is the guy that I trained while you weren't looking, he already has all the passwords and would very much like my job, I can promise you I'd be less than thrilled about it.

There are two things wrong with it, it presumes that it is his right to make these decisions and presents his boss with an impossible situation, either accept and you might have continuity, or don't accept and have an immediate problem.

Master salesmanship to get away with it but not everybody is quite as timid as his boss was there.

I can't find a better term than blackmail, but that isn't quite it because he doesn't expect any payout, maybe someone else has a better term for this, but something really isn't right here.

That decision was simply not his to make.