You're kidding yourself if you think a 4x wdc is not world class. His win in 2008 at Monza, those throughout 2009 and the come from behind victory in 2010 show that he's clearly a world class driver.
In all honesty, he needs a break to re charge. He's been racing in F1 since he was 19 years old. The demands and stress that puts on your body is not good for anybody. Lets not forget its only been recently that drivers have been picked up more frequently at 18/19 to race in f1. Previously it was 22-24 when you'd get a gig and proven yourself as a top line driver.
The other argument is that he's spent the last 5 years trying to drag performance out of Ferrari. Its all well and good when you're the dominant car, but even Schumacher would have argued that the last two years at Ferrari when he wasn't the dominant force was draining.