I guess that's why they say if you have a good thing going, don't mess with it.
I keep hearing and reading people use that reference but I've never been able to associate it with this circumstance. Both are good quarterbacks. One of them is arguably great (hope the latter that doesn't tick off some posters).
I would say the analogy applies if one of the quarterbacks were of average or poor ability. The analogy then be akin to changing out a perfectly good car tire for a defective tire or a donut. However, this scenario would be more inline with swapping out two equally good tires.
The analogy has been somewhat morphed to incorrectly align it with real documented health issues and/or past imagined (or flat out manufactured) poor play.
Certainly, Romo could affect offensive efficiency if a past or potential future injury caused his level of play to properly execute the offensive scheme. No question. However, the analogy would still only apply if-and-when any injury diminishes his level of play--not before. In short, the analogy does not properly apply for a healthy Romo.