I don’t think listing out past situations is that meaningful. There are just too many factors involved. Maybe a good chunk of the guys that sat out, were just simply bad, and no amount of learning from the sideline was going to help them.
The reverse of this argument is true too, which gets down to the total numbers don't seem to indicate that starting early hurts you at all if you just look at the results, so you probably shouldn't quote the numbers saying that they do. It just isn't true. I don't think the reverse is necessarily true either.
As you noted lots of factors involved in success and it is a fools errand to try and guess what any given player would have done in a totally different situation. You could never really know, and if the numbers don't bare out one way being conclusively better with large sample sizes than the safest assumption is probably that it simply doesn't matter long term.
I guess I’ll phrase it this way. I don’t think there is any way that sitting can HURT a players development. It may prolong it, like you’ve noted, but it wouldn’t hurt it.
On the flip side, I do think if a QB starts before they are ready, or starts behind a bad offensive line, it could absolutely hurt a QBs development. I think it’s probably fairly rare, but it does happen.
First, prolonging development is perhaps a pretty big deal. Let's say Fields takes one extra year to develop, and he's a stud in year 3 instead of year 2. You only have him on that rookie deal for a few years, so gaining one extra year of stud play on a cheap deal is really valuable from a salary cap perspectivel
Secondly, there's really no reason to believe one thing is harmful and the other can't be harmful. Maybe a guy that doesn't start gets pissed off and less motivated and bitter and then doesn't work out for his franchise because of it. Probably the case that if he's that type of player it wouldn't work anyway, but maybe it would have. Who knows?
It's just as plausible as saying a guy who's played competitive football for half his like is going to be permanently shell shocked because he took too many sacks and will never get over it even if you fix the offensive line the next year. That type of player also likely wouldn't be the type that's going to deal with adversity and be great in different circumstances.
That said, all of this is just "belief" stuff, there isn't really any evidence being put forward here. It's just people theorizing what they think might be true with no real basis (and I include myself in this group). In that vein, I'm not really going to be too upset no matter what happens. I don't have high hopes for the Bears this season regardless of whether Fields starts.
I'd rather have him get his lumps out of the way in this season which I think is likely a dead season than have to get them all out of the way next season. That said, if he starts in week 6, I don't think anything meaningful will be lost.