Largely agree with what you said but rushing a qb is not identical to stopping an intended run. Even if you are in the qb's field of vision it isn't always the best choice to break down before you get to the qb since it gives them more time and typically your main concern is stopping/disrupting the pass, not a potential run. Certainly another factor important in this situation was the athletic ability of the quarterback. Few qbs would have escaped as he did and Mitchell took a risk that didn't work out. If you are as fearful of the qb tucking and running as you are to their passing then certainly want to exercise more caution. On the other hand if Peyton Manning had been the qb, giving him more passing time is what you don't want to happen.
My basic point is that what Mitchell did was not "really dumb" as was earlier stated but a split second decision where he took a calculated risk that didn't work out.
I don't think it was really dumb. But, breaking down is an art form that if it's still being taught is not often exercised on the field. Everyone needs to adjust their risk calculators way back.
How often do we see defenders whiff on tackles that should have been a sure thing? And would have been a tackle for a big loss. The answer is every single game. Yeah, the big hit looks good on Sports Center but more often results in a whiff. When, if ever, has anyone seen a defender break down and then miss the tackle as a result? I guarantee you I can find at least 50 highlights of defenders missing for every 1 highlight of a ball carrier escaping due to the defender breaking down.
You guys are defending stuff that isn't a real problem. Yeah you want to break the pass up but look what happens when you don't break down . . . big run. If Mitchell decelerates and starts chopping his feet, Jones is going nowhere. If he gets a pass off while getting tackled, then great play, but at least Mitchell would have done his job and has to trust his secondary to make a play.