The capability of performing a 51% attack and sustaining a 51% attack are very different. Performing a 51% attack by mining a single gets you nowhere (at most you can revert some transactions, that block will eventually be orphaned).

Today i believe, most usage of Bitcoin is for legitimate purposes. There is always lots of illegal use cases for anything with value, if it has value, people will do illegal s** to get it and will use as payment. Human nature.

The energy used since the inception of the network is what makes the network valuable. Think of it this way: to create a cryptocurrency you need some "value" attached to it, Bitcoin does this by converting energy->work->bitcoin, you add "value" to the network in the form of energy and get bitcoin back (you get your "value" back in the form of money when you sell the mined BTC). Ethereum took a different approach (token sale + PoW) and now are moving to PoS.

Pure PoW seems the only way to launch a completely anonymous and decentralized crypto