Why worry? Does it solve your problem today? Don't fall into the same trap that so many people fall into who worry far too much about how popular the tools they use today will be 3 years from now. Focus on what makes you productive now. Your startup has a far greater chance of failing than Meteor.
Understandable-- but if you are productive on Meteor today, or you and your team have already committed to learning about it, I would suggest that you should see that through. This kind of uncertainty/doubt around open source frameworks is par for the course, unfortunately, and I think it would be a real shame to stop before you've at least built a prototype with the technology you've already invested time with (particularly since so much of a v1 is figuring out the interface/interaction design anyways, y'know?) Just my 2¢.
I work for a startup that has grown our team and product over the past two years, building a major application on Meteor. It's been great seeing the platform grow, and we've been able to benefit from many of Meteor's components from the very beginning. We can add new features very quickly because of the base Meteor provides.
If Meteor is a fit for your project, definitely use it. Real-time, isomorphism, and more can be of big benefits to rapid development.
