I think you are right about the starters. But I kind of disagree about their replacements. The rookie is a big posession receiver. Hill is very fast. That could work, though I think they will address this issue in the future.
I expect one of Snyder or Rachal to move to tackle to play opposite Staley when Jennings is gone (or injured again).
I really believe the offense will be much improved this year. The biggest problem last year was the terrible offensive coordinator. There is absolutely no excuse for any team in today's NFL to run one receiver pass plays with any consistancy, yet this guy did for 3 or 4 games. There was no imagination, no mixing up of plays, nothing. That will not be a problem with Martz. Will it be equal to the 9er offense of the 80's or early 90's? No way, but I do expect it to be competent at worst. It has a chance to be better than average.
I think the defense will be improved. Lawson will be back, Willis has a year under his belt, Justin Smith joins the team. If the rookie DT can play we will get a pass rush. That along with just a competent offense will improve the defensive numbers a great deal.
I think what you see is a worse case scenario. I am hopeful mine is not the best case scenario.