[andrewl-hn]

I wouldn't say it locks you. All the configuration is done via env variables, so your app code will have no Heroku-specific parts at all. Since it's a very known and popular platform there are many resources online about how to migrate away. If I were given a task to migrate between hosting providers I would rather deal with Heroku apps than anything custom and bespoke on top of DO, Linode, or AWS: heroku apps are just so predictable.

Besides, not every app needs big computing power, and many businesses can thrive on Heroku for a long time. An employee of ProductHunt mentioned on a podcast that they ran their website on 3 Performance L dynos at in 2015. That's $1500 plus some extra for the database and other services they may be using (say $3k in total). That's one of the popular websites out there, and that's a pretty good indication of an upper bound of the infrastructure cost on Heroku for a popular product. Most will never exceed the $200-300 a month.

Is $3k a big sum? Yes, it's noticeable. Will you end up spending more on hosting plus the ops work that needs to be done over time for a similar app to run and grow? Likely.

EDIT: spelling