Controversial opinion: keep Fiennes, Harris, Whishaw and (yes, even) Kinnear for the next Bond actor. All do a great job in their respective roles, even if Kinnear is somewhat forgettable. I miss the good old days of consistency in the supporting roles.
(I'm not sure why this is such a controversial opinion. It seems fellow fans keep clamoring for a new M or Moneypenny; but if it ain't broke, don't fix it.)
Can't speak for the other fans, but for me, I want a new supporting cast largely because it is broke and it needs fixing. These actors command far too much screen time considering the roles that they play. Very much want to see those parts dialed way back in future installments, especially once the new Bond is cast.
I fully agree that their roles need to be cut down significantly.
So keep the actors and just give them less screen time.
I can't imagine Fiennes or Harris refusing to do just one scene because they won't have a leading role. Pay them accordingly and it should be fine. (This would even allow them the freedom to do other roles, as just a scene or two wouldn't require clearing their schedules.) And if any future entry (hopefully far down the line) requires a larger presence of M / Q / Moneypenny, the actors will still be around and more than willing to flex their acting muscles.
As nice as that would be, I can't see EON doing it. They've pretty much adhered to the philosophy of "if you're going to get Dench/Fiennes/etc, then you give them something to do". I'd also be OK with them recasting Moneypenny and Q as well. I didn't care for the portrayal of either character in Skyfall, and while Whishaw did a much better job in Spectre, Moneypenny was still one of the weak points of the film. Not blaming Naomie Harris entirely for it, as a good portion of the blame should come down on the writers as well, but I just didn't care much at all for this incarnation of the character.