As of now there have been no restructures (moving base salary to signing bonus) or "dead years" added onto his contract of which I am aware, so I would not make assumptions that this will be the case.
I also don't see any logical reason why the team would have to "clean house" when Ben retires (I assume by cleaning house you mean get rid of all the rest of the key players and start building anew from almost scratch). It is very unlikely the team will have another starting QB anywhere close to Ben's skill step in immediately after he leaves, but I think the team still will field the best squad it can. I don't see any reason the Steelers could not still have a strong competitive team even after Ben retires. Consistently competing for championships is difficult without a top shelf QB, granted, but I don't expect the Steelers to just concede and accept mediocrity just because Ben is no longer there.