Yes, but they had one idea of their own that launched Google. That's links-as-metadata idea of indexing. It was a good idea, but nothing since then has been "from the top".
Of course not. That's not the job of the people at the top.
Their job is to look at all of the ideas coming up from the bottom, identify the winners and make sure they're getting all the resources and focus they need, and that the teams working on them are doing all of the right things. FWIW, I think that sort of leadership was lacking at Google prior to Larry's appointment as CEO. The major thrust of his management is summarized in his (rather hackneyed, I suppose, but memorable) phrase "More wood behind fewer arrows". It has annoyed a lot of users of Google's smaller, less-successful projects, but picking winners and losers and de-funding the losers is a critically important job.
And don't think that picking winners and losers is easy. Well, it's easy to do, but very hard to do right. And, FWIW, I think Larry is doing a great job. I'm particularly impressed by his decisions to kill some large projects that never saw the light of day because they weren't good for Google's overall strategic future. Those are tough decisions, especially when tens of millions have been sunk into something which turns out to be good, but not quite good enough.
As for Sergey... he's the driving force behind Google X, the research group that is responsible for self-driving cars, Google Glass, project Loon, and lots more that even Google employees haven't heard of yet. How much of it is his own ideas, how much of it is other people's ideas refined collaboratively with his input, and how much of it is him just clearing the underbrush so that other people with big ideas can get shit done, I have no idea. But they're doing very cool, forward-thinking stuff over there, and he's clearly an integral part of it.
If you're looking for whether or not their brains and skills justify their enormous net worth... of course not. Money is only loosely related to ability. Luck and persistence (which improves your luck) have a lot more to do with it. Regardless, if someone has to be a billionaire, I'm pretty happy it's those guys, because I like what they're doing with their money.
(Disclaimer: I work for Google. I try to watch the weekly company-wide meetings as often as I can, and those are the primary source of my impressions of Larry and Sergey, who host the meetings almost every week. Their obvious intelligence, insight and high standards of moral behavior consistently impress me.)