They've actually been one of the best Run Blocking O-Lines in the league that past few seasons and mediocre is FAR from what I would describe them. Hell when healthy, they're easily a Top 10 O-Line. So if anything, Portis has one of the biggest advantages with that line. Plus, if one of the starters happen to go down, they have good depth on their bench to come in and hold down the spot til one of them comes back(see 2007 season)
2010 is a little too early I think. Most, if not ALL of them should still be starters on their teams considering the fact that most of them just signed brand new contract extensions UNLESS their production starts to drop DRAMATICALLY, but I just don't see that happening with either of these players mentioned.
And you're right, no team needs to be playing 30 year old RBs with the quality of talent coming out of college, but seeing how Running Back is one of the, if not THE hardest position to learn, how many are coming out and are ready to go off the break? Not that many.
You look at backs like Portis, Edge, LT, and even Warrick Dunn. Backs who have been starting in this league the last 5 years and none of them don't look like their slowing down a bit.
I just think you're overestimating the lifespan of you being a starter in the league.