> What stops Spotify or Sotheby or any other proper company from forking off the ETH blockchain and sell NFT as a service?

It's as saying if Facebook released all of its code then suddenly it would get competition. It wouldn't.

There's the whole infrastructure behind it: thousands of nodes running the blockchain, thousands of applications running on it, developers in this sphere are very scarce because it's so complicated. If you wanted to try to create yet another "Ethereum killer", you would also need to convince developers to write apps on your chain, and people to run the nodes.

By the way, to say that Linus Torvalds is a nobody is a ridiculous thing. His net worth is humongous to begin with.