Yes exactly, transaction verification has to be profitable, and the reward prevents double spends. Mining in Bitcoin is not profitable on average, but there is a competition. However the unsolved problem: how to create a system where supply (blocksize more or less) adapts to demand? It makes sense to have a fee market, but nobody really proposed something which explains how this might work, in my opinion. A system doesn't need 5000 nodes. 50-100 nodes have enough reliability. In Bitcoin what matters anyway is the hashpower, not number of nodes. Most of these things can be much better understood from the perspective of 2008, not 2010+. Satoshi wanted the system to be as open as possible to maximise chance of success. In 2016 it is much more clear where the problems are.