The over/under on Montero’s WAR for the next 5 years is between 7 and 10. Depending on if you are a pessimist or an optimist. Catchers are a very different breed with injuries being the key issue. His BBRef age-similars, Mike Lieberthal, Jody Davis, Sammy White, Earl Battey…, were all a lot better up to age 27 than after. The best of the lot after age 27 was Ed Bailey.
Let’s say 2 WAR a year for $12 million (which is likely to be something like 3.5, 1, 2.5, 1.5, 2.0, 1.5). Not enough WAR for the buck.
Problem is you still have to play a catcher, right?
It may well be that the D’backs fancy themselves to contend, and you have to pay the players at some point. This trend of locking players up makes it seem like there are less impact players available in free agency, which probably leads to more players being locked up for fear of not being able to find a replacement. I’m not familiar with the D’back’s farm system, but it might be that they said: We need wins. We can’t buy any cheaper wins. Let’s pay our catcher.