Oh he's topped ... bottomed? ... out-stupided this idea lots of times before. The one that gets me is how, when we hit 80 or so, we should start aging backwards so we finish up as a baby. Note that Karl wasn't expressing this as a fictional concept, he was taking the stand that it's a practice we should adopt.
You could replace Ricky Gervais with any given person, and Stephen Merchant with a baked potato, and this show would be exactly as funny. Because it's all about Karl. His brain IS the alien.
The podcasts, sure. Because they learned what fans wanted, and are just there to set Karl up. They turned him into a character, even if it's not entirely scripted.
But some of the best bits from the radio show require Ricky and Steve. Rockbusters and Monkey News would be lame instead of funny, if Ricky weren't 10 mmHg from a coronary the whole time. And Steve, because he's the only one Karl can make fun of.