Also worth mentioning: it's going to take a very long time until anyone will even be able to reach the higher levels. I'm not sure about how long it takes for level 5 to reach 6 (maybe someone from the core developers can help out here... a table with that info would be nice!), and until that happens, the block reward will have declined quite a bit, and – hopefully – many others are running nodes, so the rewards are spread between many parties.