Realistically, P&O would have to reduce their fares by over £500 pp for cruise only customers, and even then, many of us would still prefer the convenience of flying from a local airport (BHX for us) rather than the slog to MAN or LHR/LGW.
I spoke to some people on our last day on board Britannia (2nd Dec) who had just paid £600pp to stay onboard for another 2 weeks - whilst I suspect this was a "one-off" to sell remaining cabins and not indicative of future pricing, that was a bargain and I was tempted!
It looks like the poor people who were stuck in Bermuda over xmas at P&O/Maleth or an insurer's expense are now finally en-route back to MAN.