IntelliJ is a good example, because you're right, you could extend Emacs to have a looks-and-works-exactly-like-IntelliJ mode. But now you've got two jobs, writing your Java and developing and polishing up your editor in elisp.

Or, for my current favorite editor, Visual Studio Code, immediately upon launching it you get intellisense via Omnisharp, really decent git integration, all the integration you'd need with dotnet core, and a ton of convenient stuff like search/replace across the whole project, refactoring that works well for C# project, etc.

Could this all be reproduced in elisp? Absolutely, I have no doubt. Has anyone put together a package to do so? Nope, though of course there are disparate packages to solve/simplify/address these things. Will anyone create one package? Nah. Will anyone bring it up to the level of polish that VSC provides out of the box? Definitely not.

Much as I like Emacs, and I really did, when I started trying out new modern editors I felt like I'd been living in a cave.

Actually, just writing this all out has made me realize that the biggest dealbreaker for me is the lack of polish. Seeing what a modern dev experience is like, I couldn't go back.