If Potter fails at Chelsea he has an impressive body of work to fall back on. He'd easily get another try at Leicester or West Ham, or even Spurs.
When Gerrard and Lampard and Ole failed they had nothing impressive to fall back on and even if a Championship club would take a huge gamble on them, they probably think it's beneath them.
So? They have still managed in the PL which is more than most managers can say they've done.
Lampard "failed" at Chelsea and was given another try by Everton. Gerrard hasn't put his hat in the ring yet (neither has Ole) and we've seen worse managers than them get second and third chances so I'm not sure why you are so certain on their unemployability.
We aren't club chairmen, and we have no idea on what they look at when making decisions on hiring. Which is why, again, the idea of someone turning down a PL offer to "build his body of work" (something that may not even work out, given the number of Championship managers who have never managed in the PL) is insane advice.
Carrick could turn down an offer from a PL club to focus on Middlesbrough, and carry them to 4th, and then maybe move to Spurs. Or he could turn down an offer from a PL club, miss out on promotion, finish 11th with them the following season, and get ignored for future opportunities. One is more likely.