I do hope for their own sake that Bubble looks at migrating away. At scale an untyped de facto abandoned 'language' is going to catch up to them sooner or later and limit their ability to iterate safely at scale.
[Disclaimer: used to work at Dropbox, but had no involvement in this project other than massive appreciation for the amount of effort involved]
Looks like they founded in 2012, which is right on time for Coffeescript. So, there's a good chance it's not just Coffee - could well be Backbone, jQuery, Handlebars etc all mixed in there, too. You can definitely work React into a codebase like that (we have) but that could be a ton of work to get away from. Those codebases often ended up as un-unit-tested spaghetti.
Codebases from that era are a tech-debt albatross. Sure, you can make them work, but at some point something's going to come along built with modern tooling and just move so much faster.
One day maybe it'll be a lucrative niche for contractor legacy-maintenance warriors.
That'll happen before I convince senior management that startups with React and Typescript can run rings around us, and that I can't find anyone fulltime who wants to work on our stack (or has even worked in not-React)...
If anyone wants to chime in with their stories about using Decaffeinate, I'd be interested!
https://github.com/decaffeinate/decaffeinate