Quickly on Zimmermann
This contract might make it seem less likely that he'll be in DC past 2016, but the honest truth is that it was never that likely. He always said he wouldn't give the Nats a discount and that the deal would have to be fair. What's fair right now in a world where Clayton Kershaw got 8 billion dollars? Anibal Sanchez had a worse resume and got 5/80 last year. 6/100 as the floor?
The Nats have signed crazy deals (Jayson Werth - and yes it is still crazy. An awesome 2/3rds of a season in year three doesn't make it suddenly a good deal) and have extended their own guys (Zimm), but they've yet to sign a pitcher long term. In five offseasons how many free agent pitchers have come to the Nats on multi-year deals? Three. Marquis (2), Soriano (3), and Maya (4 - though this is a different type of situation)
Whether he likes it in DC or not is an interesting side note but by saying the above, he's saying "pay me what I could get as a FA and I'll stay" and the Nats don't do that.
Will the eventual inevitable loss of ZNN hurt the Nats? I hate to speculate 3 years down the road but what the hell, that's what we're here for, right? Gio and Strasburg are signed through at least 2016, (and Gio likely longer given the team-favorable back end option on his deal). So the questions are (1) What happens to Fister (2) Does anyone develop into a major league starter? I assume they'll try to "Gio" Fister, meaning extend him a year or two past arbitration for a below market price. Assuming he's not dumb, #2 is then really going to push #1 (and any chance at re-signing ZNN). A good year from Cole/Giolito/Purke/etc would probably mean no honest attempt to re-sign Fister either and a projected 2016 that's Stras, Gio, Whoever was good in the minors, 1-yr free agent pitcher, gaggle of guys they try to pretend are honestly good but are more just shots in the dark. I don't like that right now but if that "good" minor league season from Cole et. al. was REALLY good, well then I might change my mind.
Right now the Nats are gambling. This is what Rizzo always does. He thinks he doesn't need ZNN because a good replacement will develop. If he's right, he's saved the team millions of dollars. If he's wrong, he at least has a chance to pull ZNN or Fister back in for a continued excellent Top 3. If he's wrong and the fact he's playing them alienates both ZNN and Fister, well then the Nats will have to face a very uncertain rotation future when right now they could cement it for the remainder of the decade for the right price.
added - on Desmond
This is more worrisome because I thought there was more mutual "let's try to stay" but Desmond is probably looking at Zimm's deal as a model and the Nats are probably looking at Zimm's deal as a mistake. Since SSs are hard to come by and the Nats don't have good hitting prospects in the minors, this is a much bigger gamble. I'd still expect them to at least to try to resign Desmond but if he comes up with another very good/ great year they've lost the gamble. They'll either have to shell out much more or lose him.