I am the CTO of <a href="http://ridewithgps.com" rel="nofollow">http://ridewithgps.com</a> which is a rails monolith that is 18 years old now. We still have original code from 18 years ago in a few spots. I am pretty biased, but I would use it for a new project in a heartbeat. It's just so easy to be incredibly productive in.<p>There are other great frameworks, but a monolithic rails app serving up an API for a react client and two mobile clients is just so dang easy to work in if you get along with it.<p>As far as performance is concerned, we will do between 1,000 and 2,000 dynamic requests a second this year with no caching during our peak season. Not crazy traffic, but respectable enough. They aren't trivial requests, depending on the user making the request and how much content their account has in it, and how many connections their content has to other users and content. We also do a significant amount of fitness / geo data processing. We'll probably be ingesting between 1,000,000 and 2,000,000 recorded fitness activities per day at our peak this year. This will comfortably fit on a couple of app servers. My performance worries are all focused at the database layer. About once a year we go in and pick all the low hanging performance fruit for 2-4 weeks, and every few years we buy some new app servers to replace old ones. Scaling has never been a real problem, but we are definitely small potatoes compared to many projects out there tackling real scaling issues.<p>That being said, I put up a PR against our frontend react repo every once in a while, and I am super jealous of the work they put in on typescript and CI. It puts some huge guardrails on development that are a productivity boost, with of course a decent up front investment in all the tooling and the requisite maintenance.