[atahanacar]

>A file can only have one group, and people use more than one application

But users can be in multiple groups. You can have files with groups like "graphics, audio" etc. and give access to the application users by adding that user to the relevant groups.

>IPC other than file based

This isn't UNIX model though, is it?

Though I agree with you. Given the current state of programs, file permissions aren't enough for isolation.