In my honest opinion, there is no such thing as preserving your engine and not driving the car to the limit. F1 is the pinnacle of motorsport and it always has the latest in technology to offer. As such, the driver of every car are constantly on the limit. If a team boss were to find out his driver is underdriving the car, he'll be off the team. You see, every worker in a racing team has their responsibilities. A aerodynamics engineer is suppose to bring the best out of a car in terms of aero design, a chasis engineer is suppose to design and make the most reliable chasis/suspension/framework, an electrical engineer is suppose to make sure all device on the car works perfectly and most importantly, a driver's responsibility is to push the car to the limit to win races. As simple as that. When a driver retires, you got to look at who's not doing their job well enuff thats all.
To some extent, i do agree at the last few laps of the race, you'll have to lower the revs to make sure your car can last but thats if you have a good gap ahead. But generally, you'll have to push, push and push to win races. Schumacher's a very good driver with some superb mechanics behind him. His wins are not all because he preserves his engine but rather the fact that he pushes the car to the limit and maintains a good gap ahead and the leaves the rest to the engineers. I'd say they've probably got the best reliability over the past 5 years and you just got to admire the team behind him.
Also, you're talking about luck too. Kimi's got all the bad luck while his team mates are normally a bit more lucky. Look at DC's retirement rate over Kimi's. He's suck a quick driver but he lacks the luck coz his car seem to break down more often than his team mate. Just like Rubens car, who seem to break down more often than Schumachers. Its luck.