The real beauty of Ray is that when Pierce, Garnett, Rondo, etc. are feeling it, it can fade to the background and just spread the floor. However, when they're not, he can still take over games. Maybe not like he did a few years ago, but he still can.
I'm not sure we can replace that. Maybe we can get a guy who could beat Ray in a 1 on 1 game; but what happens when we ask him to take a lesser role for the good of the team?
Maybe it works, maybe it doesn't. But I don't risk it on a team that can clearly win it all if healthy. Plus, if the C's sign him to a 1-2 year extension, the value of the expiring deal will still be there, but just a couple year later.