Your scheme falls apart when users want to paste complex data types between multiple instances of the same program: yet another use-case gummed up by your ill-conceived scheme. We already have the program-exiting feature and know how to deal with it. Read the documentation of the actual protocols (hint: clipboard daemon in the X11 case).
Furthermore, "just seeing a file path" is NOT an acceptable outcome from a user-interface point of view. Again, that's the kind of statement only a developer could make. You seem to imagine a world of "simple" programs that would just slurp up the clipboard contents and use them. What you really want is content negotiation. We already have that.
This line of argumentation is exactly what's wrong with the people who want to reinvent X. They look at the complexity of the existing system, imagine it can be reduced, but when they try, they either create something worse or cut features, most of which people actually use. It really smacks of hubris to suppose that we're smarter than people were 20 years ago, and that we can do a better job of solving the same problems.
X is not the problem. Copy and paste isn't the problem. This asinine buzz around replacing X pops up every five years or so (remember Berlin?), and it has the same outcome every time. It's like saying, "my web browser doesn't pass the ACID3 test --- so let's reinvent TCP!"