You don't need to resort to power law wealth for masternodes to be Sybil attacked. If they are an investment that is priced for a market return then someone who wants to also use them to spy has a competitive advantage. That being the case one should expect that every masternode in a competitive market is secretly spying. Similar to your argument about who is providing Tor bandwidth for free.

I doubt they would "destroy their investment" by spying, as with all intelligence sources the spying would be kept secret in order to protect its value.