I think we need to define 'popular'. Apple is popular but has less than 10% of the desktop market.
I can see Linux getting a 10% desktop penetration by 2019. I can see it getting even more in corporate environments looking to cut costs, especially for users who only use e-mail and web / cloud / server-hosted apps; if all the cloud advocates are right, I can even see it becoming the dominant OS in the work place. I don't see it having a majority of the consumer market in that time frame, but it will have name recognition and be regarded by consumers as a viable option for certain markets, just like Macs.
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