Just because requirements can be made complicated doesn't mean there should be no support for something simple.
The built-in .NET JSON serialise is very limited; and there are third party libraries (even used by MS themselves for ASP.NET) to address that.
I'm against super complex things, but things going into a standard library should be general enough for 80% of the use-cases in the wild (and no, I don't agree with you here), otherwise it's very hard to make changes in the standard library later, therefore instead of pushing for something half-baked, I would rather wait for third-parties to build such a library.
Scala for example has a shitty JSON parser in the standard library that nowadays nobody uses because it is limited and because third-party libraries are better. I suspect, from what you're telling me, that C#'s standard parser is in the same league.