It's not something we think about very often, but Big Ben's contract is starting to come to an end (3 years left on his $102 million dollar deal). If the Steelers follow suit, they'll extend him after 2013, with 2 years remaining on his deal at that time. How much and for how long is anybody's guess but he'll be 32 at that time and certainly they'll have to start considering drafting his replacement.
Drafting a key backup now is fine, and definitely needed, but it's not going to be all that far away before they'll need to really consider how much time he'll have left. I think we'd all be happy if Ben lasts 15 years in the league, but at the rate he's going that's no guarantee.