It's Python 3, no Flask (but werkzeug) and XML templates.
It works for hundred thkusand clients, and you can install Odoo on premise as you like. I'm 90% dedicated to that.
So... explain the "tech debt" thing, as I don't get it. You don't need Rust or microservices for every use case. Don't be fooled by marketing style "old style technology" bias and set up an account. PostgreSQL with synchronous workers works perfectly for most people.