On more thing the other didn't mention yet. The decentralized nodes can decide which software they run, i.e. making them vote for the rules of the network. This is what makes sure, that bitcoin will stay limited to 21 million BTC (it's very unlikely a majority would want to change that).
Discussion
That's a good point. There's security of the asset via mining nodes and security of the rules via the non-mining full nodes. And decentralization of both node kinds matter.