There are countries where the chances of a new mobile OS succeeding depend mostly on the ability to run Whatsapp. Simply put: WA is the free SMS in many developing contries where air-time is still expensive. Carriers even offer plans with unlimited data for WA usage and that is key to drive new sales (I know this for a fact). I still argue that the main reason FFOS on a ultra cheap ZTE failed to gain market share on developing countries was because potential buyers realized they wouldn't be able to chat with friends and family.

For this reason, if Sailfish can trully run any Android app, I'd say they have a better chance.