I do agree that it is difficult to obtain significant market share when Bitcoin dominates the market. However, there are several flaws in bitcoin, that appear with the time and increasing popularity of cryptocurrencies. For example, the issue of privacy and pseudonymity of blockchain.

If there should be an altcoin replacing bitcoin, it would be one that solves some of the issues of the pioneering currency. ITF recently published a report on privacy coins, offering quite a complex comparison of the methods out there. I think that privacy coins hold a certain potential for the mass adoption and as such, can replace bitcoin in some use cases.