Despite the fact that ethereum is one of the best and most promising coins, I do not think that this coin, or bitcoin, or any other crypto currency, can become the only crypto currency for general use. There are a lot of crypto currency functions and they are diverse, especially they are diverse among tokens. One kind of crypto currency, no matter how perfect it may be, can not replace all existing coins and tokens. But the crypto currency has only just begun to develop. Especially many tokens are now, a significant part of them are very useful for people and have specific functions. No universal coin such as bitcoin, ethereum and others can physically replace tokens.
Therefore, I believe that over time the number of different crypto-currencies will only increase.