Bitcoin still well known among crypto users, for now still no coin able to compete with Bitcoin, from the transaction volume until the popularity, and Bitcoin already being used in some stores, if the altcoin could grow better than Bitcoin then maybe it could replace Bitcoin in the future
Yes, Bitcoin is the representation of cryptocurrencies to the world and it is like a Brand or household name. For that reason and also because its investors will not leave Bitcoin, it will remain the most dominant coin for a long time to come. But I would not be surprised if we see other coins having more transaction volume than BTC or other coins having more adoption as a payment currency for goods and services, not to say that already at this time other projects have a lot more features (like smart contracts, dapps).