As a NYR and NYY fan, I agree with what you've said here. With McCann, I really hate giving up the no. 1 draft pick for him (though they can get one back if Granderson/Kuroda/Cano sign elsewhere). Both the Rangers and Yankees don't have young top level elite players in their pipeline; they only way to get those is via the draft, or selling, which both have been loathe to do. They think that New York fans would never go for a rebuild, especially for the ticket prices they charge (they may have a point with that). But from a hockey/baseball standpoint, a rebuild is necessary for the long term sustainability. If the organizations are honest with their fan base, say this is the direction we need to go for the long term, be patient, you can see the younger players grow together instead of bringing in a much of mercenaries who probably don't want to be here anyway; we'll spend at the right time, I think at least some of them will be on board; I know I would. Just my opinion.
The Yankees have a bunch of untradeable contracts; the Rangers have Richards, who if traded would kill the NYR cap wise with the recapture if traded; Nash has a no movement clause (though he waved to to come to the NYR), and his contract would be tough to fit for a lot of teams without sending something back; Lundqvist is a UFA after this year; Staal has 1 year left after this, may join his brothers in Carolina.
A full out rebuild would be tough to do for both (much harder for the NYY though), making it an unlikely scenario in the foreseeable future.