We don't actually know this but people keep quoting it as fact on the basis of the "5 year contract" quote. But then Pardew had an 8 year contract that turned out to be actually just a rolling 1 year contract. I'd imagine Power's done similar so as to cap any payout, but publicly call it a 5 year deal to publicly express his confidence in his manager and (although this sounds laughable now, he was still apparently well thought of when the contract was announced) either scare off any other club wanting to come in and poach him or at least try to ensure they'd come prepared to offer decent compensation for his contract. I think it's pride, not money, that keeps him here, on both sides of the bargain.
I think in a fairly recent interview, Williams said that the 5 year deal includes his time as a coach pre Head Coach announcement. Apologies if I've misunderstood this.
Like you say, it's pretty much down to pride. After the recent 'not a quitter' comments, he won't go until we're officially down.
