I think that there are problems with both Varlink and D-Bus. JSON is one of them but not only one. One of the problems with JSON is the data types it uses. There is no support for character codes other than Unicode (I think that "Unicode string types considered harmful"; they are not needed for handling text (even Unicode text)), and binary data must be encoded as hex or base64, which just makes it inefficient. There is other problems too, including problems with implementations (including JavaScript, which has a big integer type now but it is not the same type as numbers in JSON). I also think there are problems with the way that the message buses are working. D-Bus has some problems and Varlink has some other problems. And, it isn't true that "the whole world speaks JSON".
Yeah, using serialization format that doesn't even have obvious way of representing every possible Linux filename (nothing mandates filenames to be valid UTF-8, only thing they can't contain is NUL and '/') seems bad fit for low-level system services.
I can't wait to have apps and services crash and burn left and right due to accidentally (or maliciously) created non-UTF8 filenames in world readable directories.
> Yeah, using serialization format that doesn't even have obvious way of representing every possible Linux filename
That is hardly the only problem with such a serialization format, although it is one of them.
> I can't wait to have apps and services crash and burn left and right due to accidentally (or maliciously) created non-UTF8 filenames in world readable directories.
It is not only accidentally or maliciously. Sometimes it may also be done deliberately because it is a character set other than Unicode, without the intention to be malicious.
(A user might also deliberately want to display file names using a different character set, perhaps in order to ensure that all characters can be displayed unambiguously. Of course, this is not necessarily what everyone intends, but some people will.)
