For me it's not even about mobility, though all things being equal that's a nice bonus. From what I've seen I think Fields is a better pocket passer than Mac Jones or Trey Lance. He's got more arm strength than Jones and he's more accurate on deep balls than Lance. I was initially nervous about Fields because I saw some of those egregious mistakes when he held the ball too long or forced passes into tight coverage that just weren't there. But having watched a lot more footage now, he's beat the pressure more often than not and usually with his arm not his feet. So I see those as correctable flaws whereas Mac Jones lacking zip on long throws or bypassing better reads to hit the check down could lead to him washing out in the NFL. Trey Lance is a super athlete but he didn't face strong enough competition in college that I feel I can draw too many conclusions from his tape. We gave up a lot to roll the dice on those kinds of question marks.
Personally I think only one of these three is an NFL QB. I also think Shanahan knows this. He's been through two epic 4th quarter collapses in the Super Bowl now so it should be as obvious to him as it is to us that you're not going to be able to scheme receivers open consistently in the NFL. Eventually you need a QB who can find that small window and deliver the ball on target before the defense can react. I have no idea what is going on behind the scenes and how much of this is just the usual talking heads hot air we get around draft season in every sport. In general I feel like Shanahan is the offensive guru, he should know better than I do what skills will translate in his offense. It just makes me nervous when I watch the tape and one of these guys looks like he's night and day better than every QB in this draft not named Trevor Lawrence and somehow that's the guy we're being told is not under consideration. It's just very strange.