even laroche is putting up .259/.339/.445 while not having his best year, that's slightly better than your uninjured espinosa line. I don't think WAR is necessarily a fair measure here since you're talking about shifting positions, you're inserting a 2B, but the bat he's ultimately replacing is a 1B, so the positional adjustment probably should be backed out to make the comparison fair. You're not necessarily talking about a big defensive upgrade either- yes espinosa should be an improvement over Rendon, but how does Zimmerman compare to Rendon at third, and how would Zimmerman learning first compare to a first baseman?
Rendon is playing average, at best, defense at 2B while Espinosa plays Gold Glove caliber defense there. Rendon plays excellent 3B defense while Zimmerman is playing below average (at best) defense there. Further, your post makes a number of assumptions, among them Zimmerman actually being able to play 3B adequately after LaRoche leaves next year, as well as the fact that we can find someone to plug in at 1B to even replace a reasonable uninjured Espinosa-level bat - say .250/.325/.425.
I'm trying to point out that kind of bat is not exactly easy to find. There's no guarantee Tyler Moore can give you that, Chris Marrero, Matt Skole, etc. There's no guarantee you can get that on the free agent market if you're not willing to give someone 8 years and $200 million. Otherwise at 1B you seem like you're stuck with a guy that looks an awful lot like Carlos Pena.
This isn't the steroid era anymore. If this is your infield, you're doing very well offensively:
1B: Zimmerman .825 OPS
2B: Espinosa .750 OPS
3B: Rendon .850 OPS
SS: Desmond .825 OPS
Not to mention that should be a fantastic defensive infield. Now if Tyler Moore/Matt Skole can OPS .850, maybe it's worth having worse defense at 1B, 2B, and 3B, maybe not. But we don't have a ton of boppers ready to come up and be that guy.