Eventually bitcoin will be overtaken. Though not in the next couple of years.
Bitcoin won't get overtaken you coin jumper. With Lightning Network coming closer it's going to be a challenging time for altcoins - which ones will become even more redundant than they already are, and which coins with an actual purpose will manage to keep growing. I wouldn't want to hold any shitcoins with this in mind.
Bitcoin will still be the top cryptocoin for now but expect the alts to have their fair share of the market.
The problem with people paying attention to "Bitcoin's dominance" that's based on an utter nonsense metric as market cap, is that they don't understand that Bitcoin can ONLY lose in this aspect. It's the entire altcoin market versus just one single coin called Bitcoin. Do you seriously think it's fair for people to keep referring to Bitcoin's shrinking market cap dominance?