There's way too much myth surrounding Kimi, for the good and the bad, some stuff I find difficult to agree with based on simple evidence.
1) This "Kimi is fast only when the car is right" is absolutely not true... If there was a car that wasn't "right", it was the 2003 McLaren, and Kimi pushed Schumacher all the way to the last race of the season, when drivers like Montoya were left behind. This was no mean feat, although Kimi (and most other drivers, truth be told) benefitted from the new score system that year, which lessened the value of Schumacher's superior win tally. It's necessary to point out, however, that Schumacher and Ferrari were helped by the FIA banning Michelin's wider front tires, crippling Williams, who looked like the favorites mid-season. In what was the third best car, Kimi was runner-up at the end of the year, not bad at all. Kimi's performance with the 2009 Ferrari after Massa's unfortunate accident was also pretty good, including a race win, and that car is widely recognized as the worst one in recent times (yes, worse than Alonso's 2012 ride) and a horrible one to drive. He's bashed a lot for his stints at Ferrari, especially by Alonso fans, but the truth is that Kimi prevented Ferrari from going winless in their worst season in recent years, and should've gotten more recognition for that. Instead, he got a kick in the butt and a fat paycheck to go with it.
2) He seems to be a smooth driver in the vein of Button. You cannot supposedly "handle more oversteer than Schumacher did" without being smooth. Understeer is the enemy of the smooth driver. This made him struggle in 2008, because of tire temperatures. Massa did not have his problem, as it was reported his driving technique was quite erratic, and, if you bludgeon the car around the circuit, tire temps will rise. It's also part of the reason why the Ferrari was catastrophic in the rain that year, and why Massa himself has always been a terrible wet weather driver (especially that year).
3) Drivers do have their own styles, which effect development. Motor racing is ultimately an affair where the car not only needs speed, but also consistency. This consistency is achieved with driver skill, but also confidence. Keke Rosberg wasn't exactly a scrub, but reportedly couldn't match Prost at McLaren because their driving styles contrasted too much. If you don't trust the car, you simply can't push it hard enough.
All being said, I simply can't expect a driver nearing his 40s to perform as well as he did in his prime. Kimi probably lost some valuable seat time in the years he was absent from F1, although his sabbatical's effect lessened as he got used to the cars, of course. Mansell in 1995 didn't unlearn how to drive an F1 overnight, he was simply unfit: his time was up. Prost in 1993 was kept honest by Damon Hill at times and had much more difficulty winning the title than his car's superiority would suggest. Lauda wasn't the same driver in 1984 (when he last won the WDC) as he used to be in his prime either. The list goes on.
Perhaps it's time for Kimi to hang the helmet, but that's not really a problem since there's plenty of racing to be done once you leave F1 even when you're almost 40. I'd like to see him at Le Mans, myself, though he's a Finn so he's devoted to rallying.