Bears have poured everything into ramping up Caleb.

I'm guilty as can be on this myself.  Full disclosure, I admit that I was on the "keep Fields bandwagon" too.  I blame it on the fact we've not drafted a great QB1 in so long that I'm half-nuts wanting the team to get it right with a rookie QB.  The idea of sitting a guy seemed good to me back then.  But now it makes sense to do what Eberflus is doing with Caleb.  Especially with the rookie contract benefit.  But, this is for 1st round guys who have the gears to actually do this.  Nothing wrong with letting a promising project QB develop - a guy like Steve Young (played initially in the USFL) comes to mind.  For that matter Joe Montana was a 3rd round draft pick who sat for a year and a half.  And it's probably a long shot but Bagent comes to mind as a possible starter someday.  Bagent's floor seems to be a solid backup guy.  But there is a chance he could be a starter in the league someday too.  

This is because many fans overthink things and think there is some super secret formula to producing a great QB. The reality is through luck or skill you have to pick the right one. The only real determiner to sit someone or not is who gives the best chance to win at that moment. Mahomes sat because Alex Smith was playing at a high level. Love sat because Rodgers was playing at a high level. And Rodgers sat because Favre was playing well. Had the rookie QBs had different lesser QBs in front of them they all would have likely played their rookie years. And I don't think there career trajectories would have been much different. 

Even after Nagy would have changed his mechanics?

Nagy had his faults no question.  His scheme was overly complex and resulted in a lot of false starts, wasted TO, delays of game, and so on.  He was also rigid an inflexible in his playcalling.

Fields also has a lot of flaws.  His windup/delivery are SLOW.  His progressions are SLOW.  He doesn't throw with anticipation well enough.  He gets happy feet way too easily.  And so on.

Seems like both of these guys have "low ceilings" so to speak.  I've noticed that Nagy hasn't exactly been a hot HC candidate since leaving here despite the fact he's technically the OC of the league's most successful team.  No one seems interested in giving him another HC gig.  Maybe he's just hanging on waiting for Andy Reid to keel over.

Likewise, Fields has a golden opportunity with Wilson hurt and/or sucking to get another #1 job and seems to be fumbling it away figuratively.  In no small measure because he continues to fumble the ball away literally.  Maybe he can become a high-end backup but he doesn't look like he's ever gonna be the franchise starter we all hoped he would be.  I finally gave up on him last fall.

Re-watch some old Fields replays and watch some of those many WR screens Getsy called.  Watch how slow JF's delivery is and watch how often the ball isn't even well-placed for YAC purposes.  And the whole freakin' point of a screen pass is YAC.