I can't think of a single billion dollar Corp that can run Rails on a few AWS boxes. Time and again companies have rewritten systems from MVP to handle data safer, better, and faster, from Dropbox to Twitch to CloudFlare to Uber to Mail.ru...

Then there are plenty of 7 figure companies that have also had scale issues. Game companies come to mind first and foremost but they're hardly the only ones.

You want to run your marketing website on Rails? Yeah of course. But billion dollar companies on Rails?