There just isn’t a C#/.NET plugin for IDEA that replicates the Rider functionality, though. Same with CLion and AppCode. They do ship Ruby and Python and PHP plugins. The JavaScript/web dev stuff is baked into all of their IDEs.
Which means IDEA does not contain all the features of the individual IDEs.
The thing is, I’m confident in saying the plugins don’t replicate 100% of the feature set of PhpStorm or WebStorm either, but I’d defer to someone from JetBrains who has first hand experience.
If you're confident, what features don't they have?
I subscribe to all their products so I can verify this :) I'm not sure why they don't have C# or C plugins but my guess is that it's because there's enough extra work in those IDEs that they want more money for them. Rider contains the ReSharper backend, which is a completely different application from IntelliJ.
There are differences in default config, and sometimes the independent versions have a faster release tempo, but it's the same language code.