I will go ahead and say the part that Eisen didn't say. In addition to "if it ain't broke, don't fix it" look at who you have. Brock has done everything Jimmy did to the same level Jimmy did it. But in addition Brock has added mobility and creativity to the equation. If the play didn't work the way it was drawn up or pressure blew things up Brock could move and buy time and he could improvise with the time he created. Jimmy has been rarely (if ever) been able to do this on a consistent basis. With Brock it is like they have Jimmy who suddenly figured out pocket presence, mobility and turning nothing into something.
That is why you continue to play Brock.
When it comes to next year everyone can probably agree the chances that Jimmy would be back are pretty much nil. So you are looking at Lance vs. Purdy. The question boils down to does this system value consistency and making the play vs. a stronger arm downfield. I think that anyone who really looks at what this system is about realizes that it is all about distributing to the great playmakers in the offense and keeping on schedule. So does a big arm with a .549 completion percentage make more sense than less of a downfield arm with a .671 completion percentage in this offense? Does a 5:3 TD to INT ratio make more sense than 13:4 in this offense?
As a side note, I was surprised to learn that despite Brock's speed being more in the area of quick first few steps rather than top end speed, Brock dropped a better 40 time at his combine (4.84) than Lance did (4.92).