What happens if one actor controls a majority of the nodes?

There’s a lot of information available on the threat of a 51% attack via mining Bitcoin, and why that is so costly and difficult to achieve. But what if nefarious actor were to control more than half the nodes? According to Bitnodes.io there are only about 18000-19000 reachable nodes on the network. Given that it would only cost about $10million to spin up 20,000 nodes, can someone explain to me why that isn’t a threat to the sound operation of Bitcoin?