DEAR MISS MANNERS: I have never heard of "Call or write if you do not wish to attend" when I receive an invitation. I have always understood RSVP to mean "Let us know by such and such a date if you plan to attend."
There is no mystery if you do not reply by the date mentioned, as that automatically excludes you from the invitation list. No muss, no fuss, no need to write or call and say you cannot make it. The host automatically knows that all who plan to attend will have sent in an RSVP by the specified date.
It is also more "feelings" friendly, as you need not offer an explanation as to why you choose not to attend. You may not like the people, or you may have a previous engagement. By not replying, you do not have to hurt or ruffle feelings.
It adds no burden whatsoever to the host, as by the deadline, they know how to plan their event, as they know exactly how many people are attending. It saves time (and maybe feelings) for both the invitee and the host.
GENTLE READER: Do you have any idea how much havoc and hurt you are causing? You and everyone else who hold the bizarre and callous notion that people who are good enough to offer you hospitality may simply be ignored?
Even the most casual invitations require definitive replies. If a co-worker stopped by your desk and asked if you'd like to go out for coffee, would you just turn away without a word?
About the workability of what you suggest, Miss Manners assures you that you are wrong. She is flooded with mail from anguished hosts who find that people who have not responded to their invitations may nevertheless show up -- and those who have accepted may not. (The same is true of the odd but common directive "Regrets only," which you claim does not exist.)
Hosts' feelings are also something Miss Manners knows about. People who entertain are not so naive as to believe that everyone can or will attend. But they are insulted not even to receive the courtesy of a reply.
No excuse for declining need be given -- just an expression of gratitude for the invitation and regret at being unable to accept.