Miller's cap hit is high, but he's probably the best blocking TE in the NFL that actually goes out into routes. Miller is tough, has great technique, and is a willing blocker, which is something that is tough to find coming out of college anymore, with teams splitting TE's out wide more often than not. He's the kind of TE we need to make our scheme work, and it's also why McCoy took a couple of years to adjust.
Miller doesn't have to be fast. He stays in to block, then runs out in the route, which gains him a ton of separation. Plus, he's a smart player who knows where to find holes in the zones, and he has some of the best hands on the team. McCoy gets open many times because we run Miller on shallow drag routes, he sucks up the LBers, and McCoy is wide open.
I don't know if we can afford to keep Miller, but we need to. McCoy is getting to be a better blocker, but he's still not physical enough to match what Miller does for this offense.