There must be a misunderstanding from your side. Running a node compared to mining is a different procedure. The system's decentralization is maintained due to the increase and the distribution of the hashrate. Whether we had ten or a million nodes, but the same proportion and distribution of hashrate, it'd be the same thing.

If you run a node, but do not vote for the security of the network, then you're just doing it for your own good. Thus, you may not even need a full node, but rather a pruned one, which comes cheaper.