> the default Heroku server

There is no "default Heroku server" you get exactly what you specify. If you specify nothing you get what Rails runs by default which is webrick and happens to be awful in production. It's not that "Heroku has a bad default" here but rather Rails' default web server is not intended to run in Production. Most of our docs try to emphasize that you should be using another server other than webrick. Some people want to just bang out a proof of concept and not worry about configuring Puma or Unicorn, for them webrick is fine.