also - while AWS can make infrastructure convenient to scale up, rarely is it cheaper. It certainly can feel cheaper in the beginning as its pay-as-you-go, but averaged out over N years it's not. AWS also has reserved pricing to aid with this, but most startups are not in a position to commit to either real hardware or 3 year contracts up front.
Most startup's spend an enormous amount on marketing to get any traction. I'm sure there is more examples like Slack that did not use much marketing, but they are very rare.
If you build a startup and hope to iterate your way into being viral, this is bad planning in my opinion, no matter how awesome what your building is. Unfortunately, one that I had to learn the hard way.
It's a great way to alert a much bigger competitor of an emerging market so they can eat your lunch, though.