My laptop is a tool I use every single day and have with me almost all of the time. I do not need internet connectivity to remain productive, which is a huge boon to me when travelling. An IDE that requires constant, reliable connectivity to work is therefore of little use to me.
Which could very well be a determining feature in where people host or which Git provider they use.
That may be true in theory. In practice "essentially" is an awfully long way from "absolutely", with offline support in many apps being either choppy or non-existent. Even the venerable Google Docs is something of a ####show when used offline, to the point where I gave up and went back to Office 365.
Kind of like how you can already edit files and make a pull request in the GitHub web interface, but you can also run the program and check it works!
I've tried Cloud9, and found that the plugin issue was a killer. If a Web-based IDE can't use an existing ecosystem of add-ons then the developers have to implement linters, debuggers etc. for every language, as well as things like build tool support and version control integration. For this reason, Cloud9 doesn't actually have the functionality of an IDE or a programmer text editor, and I could not imagine asking professional developers to adopt it as their primary environment: it is closer to Notepad++ than VS Code.
Web IDEs in general are a great concept though, mostly because developers generally love having their editor customized the way they like it, and from there, having it in the cloud accessible from any computer is a huge win. That being said, it's hard to compete against the plugin model that Sublime/Atom/VSCode have.