Ah, well this is an entirely different question to the future of Sailfish. Because the Sailfish UI is not open, it is illegal for you to distribute the source code without Jolla's consent. And this illegality survives the end of a company -- whoever ends up holding the intellectual property still has control over the source.

This is not a problem for Mer, which is open. It doesn't really matter if everyone working on Mer leaves the project; you can still pick it up yourself, use it, modify it, or distribute it without being sued. (So long as you follow the terms of the open license.)

Of course, if your only worry is about how many people are actively working on the project, may I ask: what is wrong with iOS or Android? Those OSs have enormous numbers of folks working day and night to maintain and improve them...