Also, while they can't be removed since they are written in firmware, they certainly can be spoofed using software. So a MAC address couldn't even effectively be relied on.
The technical side of routing doesn't matter to the request though. The request is for browsers to provide a token to uniquely identify an instance of their browser. Which will never happen because that lessens user privacy which is of the up-most importance. There are ways to handle what you'd like to do, it just involves more effort on the developer side. The order of importance goes like this:
- End users
- User agent vendors
- Specification authors
End user needs take precedent over all others. In this case, it doesn't matter if it makes developers lives easier, end users could tangibly be hindered by the addition. Therefore, it is something that won't even be considered without some serious data showing end-users won't be any worse off then they currently are and that the tool requested provides enough benefit to developers to use it regularly.