Yes, both are theoretically possible. Now let's imagine that someone decided to take control of Ethereum and began to secretly buy coins in order to start controlling enough of them. Once he got them under his control, nothing more can be done, because who controls more coins, the same and more influences the network. Including when voting for soft forks.

If someone has collected enough hardware under their control to control bitcoin, at any moment someone else plugs in even more hardware, and control disappears. There is no way to establish a monopoly in almost any way.

This is one of the reasons why Bitcoin is more decentralized than Ethereum.