Bitcoin currently suffers from a serious scalability issue with no solution on the horizon. Ethereum constantly gains greater acceptance and hosts an increasing number of projects on its blockchain. When will the inevitable occur? My opinion is that in about one year the marketcap of Ethereum will be greater than that of Bitcoin.
I don't think that any other coin will ever surpass a bitcoin. A bitcoin is a father of all crypto. Maybe after very long time when bitcoin will become as old as a dinosaur it will be replaced with something new with a high-end technology.