I'm another one who thinks their genuine chemistry and friendship helps them a lot. I think even the projects that weren't so successful probably still did better due to their involvement than they would have done if other people had been in their place.
Alien Autopsy was another thing that didn't do too well wasn't it?
I think the thing with them is they are bigger than any show they do so they get away with a flop here or there while other presenters and acts might not be so easily forgiven.
Now as long as they stay away from crime and making offensive comments a la Bill Roache, hopefully they'll be around for a good while!