I think experience is less of a problem than his accuracy, the competition jump, and the year off of football. It's a recipe for late, inaccurate passes, which turn into pick sixes.
For the lack of experience he had, he was running as close to a pro-style offense as you'll find in college, and Shanahan has loads of experience with "Jimmy Gimmies," those passes that the mediocre QB can execute without having to read the defense or throw more than negative three yards down field.
So, I think if Lance learns the offense, his experience isn't as much of a problem as some think, because the offense he'd run would be no more difficult for a QB than what Jimmy runs (which is to say, not very).
However, if you watch his tape, he has inconsistent accuracy, and THAT is a big problem. Because what's the point of putting in Lance because he has a bigger arm if he just misses the throw? Might as well keep Jittery Jimmy in there. And in fact it would be worse early on, because the game is much faster in the NFL, and he'll be late a lot, most likely.
On the other hand, he's proven to be a good decision maker, but I suspect that would just mean he takes a lot of sacks early on instead of throwing stupid picks. Which of course is worse than having Jittery Jimmy out there.
But the biggest reason Lance needs to sit is to fix his fundamentals and improve his accuracy.
