Let's take a long stroll down memory lane and look at the starters (not including those who only started a game or two due to injuries or whatever):
Bryce Young.
mac Jones.
Tua.
Hurts
Coker,
Sims,
AJ,
Gmac,
Jp Wilson,
Croyle
Pennington,
Watts,
Zow
Phillips (replaced by Zow after 4gms)
Tucker (partial year)
Kitchens
Burgdorf
Barker
Woodson
Hollingsworth
Smith
and that's as far back as I have direct memory of.
out of all those, who would you rate behind Milroe?
probably Watts who was only playing because Zow kept getting hurt, but at least Watts was decisive running the option and made the right keep/give decision most of the time, so maybe not.
Phillips - he got beat out.
Pennington? he also only played due to injuries then quit to play baseball
Burgdorf? Only playing significant time due to injury
Kitchens - maybe/probably.
Woodson - maybe/probably.
The rest: All ahead of JM I think. That's not to say they all have as strong of an arm, or could run anywhere close to as fast, but they'd at least read the field, make audibles at the line, know the box count, and mostly (no one is ever 100%) get us to the right play and make the right decisions on the field. Even Kitchens and Woodson were probably ahead of JM in stuff pre-snap; they just struggled with what happened after that