Altcoins generally are just a bigger market, but for most part, Bitcoin is the bridge between them. Generally, if you want to buy Altcoins, you have to buy Bitcoin and then use that Bitcoin to purchase the Alts. It's hard to find places to buy Altcoins directly. I believe this is what makes Bitcoin and Ethereum in particular so large in terms of all of the other Altcoins. As long as the system of having to purchase Altcoins using Bitcoin/ETH exists, it is very unlikely that the Altcoins will surpass Bitcoin, as there will be people putting as much money into BTC as they do Alts.