At Replit, they were looking for a seamless experience for their users, focusing on improving the stability, performance, and accessibility of their editor. After relying on Monaco for years, they found it lacking in mobile support, had a clunky API and poor documentation, made customization difficult, and was heavy and slow due to its large bundle size. They replaced Monaco with CodeMirror 6, which offers a lightweight, performant, and highly customizable editor with excellent documentation and a modern extensible API. The transition required significant effort, but Replit believes it's a better choice in the long run. They're also heavily involved in the open source community, contributing code and financially supporting CodeMirror and other projects like Emmet.