I think it’s easiest to explain to newer users basically as, Meteor can be adapted to be completely self sufficient for nearly any application you will realistically have.
Of course as you grow you will have to assess the problem areas and optimize, but that’s true for any application.
If your app does somehow achieve insane amounts of traffic that Meteor can’t handle - you would be having similar issues no matter what platform you choose. That’s when you put those big bucks you made with that traffic to work and start working on a completely customized solution for your specific problems.
You can see an example of this in the Uber link shared above (here is the link again: http://highscalability.com/blog/2015/9/14/how-uber-scales-their-real-time-market-platform.html) - they mention how they eventually hit limitations of their old system and had to improve it with a new system. This is exactly what you would need to do if your application ever got to the point of “crazy traffic”.