Harvey is always a dick and he always gets what he wants. This time around Mike is, or at least was, his boss and Harvey still treated him like his water boy. So if Mike is being a dick for asserting himself, then good on him for being a dick. It's time Harvey takes a loss.
Harvey told him how he should approach wanting to take over another firm. Mike ignored the advice.
Harvey doesn't get to tell Mike what to do anymore. When that occurred, Harvey was Mike's employee, as in, his jobs is to help and advise Mike - not tell him what to do. Mike tells Harvey what to do - or he's fired, which he was.
honestly this just makes me wish they could both somehow lose.