Moving Zim to 1st is a bogus argument. He is 27 this year. Sure he is fringing "injury-prone" but there is nothing to say that his footwork and fielding ability have really declined as a third baseman. He is an elite player at a position with very little elite clientele. I wouldn't even think about moving him to first until he was at least 33-34, which is six years away AT LEAST. Also Look at Scott Rolen who is still playing GG caliber defense and can still hit at the age of 36. I don't see a move to 1st for Zimmerman based on ability, only if Rendon should force him there because he can't play any other position. I like Rendon at 2nd though and think he could easily turn into a guy who hits like Uggla and fields like Utley. That is all speculation though and I think not signing Fielder is a huge mistake especially if you base it on "what if" prospects and "what if" payroll projections in the future.
Sign the guy, go to the postseason and watch the fan base come out in droves, well hopefully on that last one. Bring on the best looking Murderers Row in years and top it off with an up and coming power pitching rotation and we at least have the stamp of being a true Washington Senators like team!