May be the case - but in my experience, issues of scale don't creep up on you - one day hit a hard limit and your service is crashing all the time.
.. And if you haven't thought about it - this can happen at any volume. I've seen apps that test great with 5 users and then bomb when you get 3 more.
Just my experience. Not saying as an overall strategy it's not a good idea - as a business this might very well be an effective model - just saying that ignoring it technically is a bad one.
Campfire has handled 10 million messages since it started in 2006. That sort of traffic could be handled on a 386.
Their strategy is getting people to pay for simple stuff. That's a reasonably small market.