I don't think Doc is planning on leaving. I believe he wants the same core back, and this is his only leverage to get that. Ultimately, I do think he will be back. I know many do not, but I say give the core, with Doc, another chance at it, hopefully they stay healthy. PP and KG can play reduced minutes. A second championship would assure both KG and PP a place in the rafters. (I feel they deserve it anyway!) As I have said before, IMO the team is already well into the rebuild mode.
If it does not work out, PP, KG or RR may be worth a lot more to a team trying to find that final piece to get them over the hump at the trade deadline... either as a player or as an expiring contract or a player who could be waived for cheap.
If we don't bring them back, we are probably not going to win a title this year anyway, and they may be able to do it together.
As far as the "underachieving" Doc comments, I believe just the opposite. Doc has taken teams that were injured, duck taped and patched together far into the playoffs...sometimes to the finals, and I do not think there is another coach in the league who could have done better with the teams Doc had.