It would be if you gave him away for modest picks or longshot prospects. A rebuilding team like Buffalo would be pretty silly not to take Howard. He's price controlled and reliable, just what they need.
If you're getting Miller, who cares what the return is. An upgrade is an upgrade. Just pretend you didn't sign Howard to the extension. Tada. And yes, I know many dispute Miller is an upgrade. Having followed both over the years, Miller steals way more games. And like I keep saying, I'm actually a Howard fan. But I always like a chance to get better.
Finally, I think it's worth noting that while Holland has been loyal to a fault with his skaters, he has been quite liberal with swapping goalies. Perhaps it because he was one? I'm not sure. But was there a period in Detroit history, before Howard has settled in, that the #1 spot was a sure thing? Every off-season it seemed like Holland was scouting for a better goalie. Osgood, then Vernon, then Cuju, Hasek.... back to Osgood!