I think Kobayashi is a driver who rises and falls based on how good the car is. He started the season brilliantly, when Sauber had a good car, and fell off when they fell behind. Remeber when the car ran well in the wet, he was 2nd in Canada, and fell back massively as it dried and the car stopped working properly.
I think he’s a bit like Jean Alesi. Generally popular, a great racer, very quick but not quite the complete package you need to win consistently. I can well see him wining if he gets a winning car, which is more than you can say about any other Japanese driver!
As such, Sauber, with few options anyway, would be mad to replace him. I don’t see any better drivers on the market, and even fewer who would take a Sauber drive. If they produce a brilliant car (and stranger things have happened), we’ll probably see a new, improved, Kobayashi.