Most of the altcoins start with a crowdfunding and with investing (ICO) at most of these funds are accumulated in bitcoin.
By this not only that these altcoins become popular but also bitcoin become more popular because through bitcoin they are investing in another cryptocurrency.
Indeed. Maybe doubling bitcoin could be possible in investing with altcoins. They have been looking for a way to double it, it might not be that fast but it could happen if the invested alt will really shine.