So, I have always disagreed with this opinion. I think I want a QB the exact opposite of Fields as his backup. Your backup isn't going to be as good as Fields at anything, so what's the point of being the same offense, but worse at it? Everyone points to the Ravens, but conveniently leave out that Huntley went 1-3 as a starter last year and they had the 1 seed in the AFC when he took over the job. He's 2-1 this year, but they average a full TD less per game with Huntley in there. Despite him being a 4th pro bowl alternate, he's nowhere near as good as Lamar.
If Fields is going to be hurt long-term, you're screwed anyway. You're going to lose a lot of games. If he's hurt short term though, you have a chance to catch a team by surprise by throwing out all of their scouting reports, films and changing your tendencies. For example, Fields likes to hold onto the ball, throw the ball deep and is a run threat. You throw in a guy like Siemien and now the defense has to adjust to a guy that's going to get the ball out quick like he did the 1st drive vs the Jets. I get the argument about not changing the playbook, but this is the NFL. Every team has 100s if not 1000s of plays. The concepts are all similar. I think there's much more value in a game or two with running a completely different offense, and letting your playmakers adjust on the fly instead of trying to run the same stuff the defense is prepared for from watching film, but with a lesser player running it.