I agree that injuries limited Aikmans mobility twords the end of his career but really, the thing that limited his mobility most was the scheme. There's a reason we brought Norv Turner to Dallas.
1. I think the franchise recognized that Aikmans willingness to take punishment would eventually catch up to him.
2. Aikman's arm strenght and accuracy, combined with Irvin's ability to catch, use his body and over all competativeness really fit Norv's timing offense.
In that offense, the drops are precise. They are bang-bang type executions. You want your drops, your routes and your deliveries to be very quick with little waisted motion. That offense does not allow for QBs to move in the pocket very well. Because everything is prediciated off everybody being at exactly the right place at the right time, the ball coming out at the right time and on target, you don't get a great deal of deviation in drops. You don't see a lot of roll outs.
The QB stands in the pocket, ideally, and throws to a point in the face of pressure. You don't avoid the pressure and try to make things happen off the scramble. Rather, you beat the blitz or pressure with timing. That's why Aikman was so difficult to sack. The ball was always designed to be gone before the pressure could get there.
Very few seven step drops or long developing plays. Lots of 3 and 5 sets in the passing game. Lineman needed to be big and strong rather then quick and agile. Reason being, you design protection to last only x seconds. Theoretically, the ball is gone in x seconds so anything else is wasted movement. You want the defense to have to run around the blocks rather then threw them. The QB would not be scrambling so mobility was not a premium. Strength to disrupt the initial path to the QB and the ability to move anybody, regardless of size or speed, a certain amount was key in the passing game.
The offense, IMO, was the real limiting factor in why Aikman was not very mobile.