Sami's pace was not the issue, it was his loss of swing with the new ball and loss of reverse swing with the old ball. When he first started his career from 2001 to 2003, you see him operate at 136-140 km/hr but he could swing the new ball and then reverse swing the old ball and that was when he was a threat to the batsman. It was 2003 onwards that Sami started to crank it up to around 144-155 km/hr and unfortunately that was his downfall where he focused more on speed and completely forgot how to swing the new ball and reverse swing the old ball.
I compared his bowling action from 2001 to 2003 and from 2004 onwards and its like he literally developed "hips" similar to Steven Finn where he forgot how to use his wrists to get swing and with the loss of his swing, his pace whether he was bowling at 136 or 148-150 km/hr plus became irrelevant. Plus he was not consistently quick like an Akhtar or Lee anyways. He didn't have a good consistent bouncer and the loss of swing killed his bowling and ofcourse being mauled every day in International Cricket, habitual failure takes a big toll on your confidence and self belief.
He confirmed to Pakpassion in an interview that he himself decided to cut down on his run up to stay injury free. I remember an Osman Samiuddin article where he made a very telling point where he mentioned that Sami never really had the desire to be a succcessful international cricketer, he was someone who just liked to go with the flow and treat things like a day to day job, most Pakistani players when they get dropped from the team, they either whine in the media, lobby with their favorite sports journalists, go to meet the PCB chairman, the selectors, lobby with their domestic coaches as part of efforts to get back in favor and into the team, while it was commendable that Sami who never did these activities ever and went back to Domestic Cricket, it is also proof that perhaps being dropped from the national team never really hurt him, that he was not hungry enough to be successful in the game and that he felt that he was good for nothing but domestic cricket basically.
Biggest tragedy in Pakistan Cricket. Even Imran Khan tried his best to help him but to no avail.