Everyone has always said that when Bitcoin becomes so easy that grandma can use it, it'll go mainstream. But the problem is securing Bitcoin, which the average person cannot do all that well, unfortunately. :/

That being said, Steemit doesn't require much knowledge of crypto-currency to gain access to it. That means, even grandma can make money on it. So yes, it'll be the gateway crypto. I just hope that people diversify some of their funds into bitcoin. :)