I think the Steem technology with be serious hardcore, but i dont think the platform will become mainstream in the coming years. Its going to happen like Myspace and then Facebook and the social media explotion...
New technologies based on Steem will take the market. Like Appics, but the Steem coin will rise as those new Social Media Tokens will be worth becouse they'll be based on Steem.
So... my especulation is: maybe Steem will keep growing two or three more years... of course: if no mayor developments comes to the platform from the designers.