Many many businesses can survive on a $25 a month 1x dyno and the free database tier (and free sendgrid, free newrelic, free whatever other addon) and can be pretty sure their site will basically never go down.
It's a fantastic security blanket, but...as your business grows you'll start to pay. The question then becomes "If I do all this myself on AWS (or azure or gcp) how much time am I going to be sacrificing time against building my business dealing with random infrastructure crap?" Or "at what point does it make sense to hire someone to focus on all this infrastructure and how much would that cost vs. just paying for heroku?"
