In my original post, I didn't advocate mandatory fitment of radios to ALL aircraft. What I did say is that I believe they should be mandatory to attend fly ins where high performance aircraft will be doing flying displays.
However ...
Seeing some of the tissue thin justifications put forward in some of the posts for NOT fitting radios, I have come to the position that if you're flying around out there without a radio, I don't want you anywhere near me.
I accept that as a PIC, I must keep a lookout for other aircraft, and I do. But I expect the other pilots to do their part to help me know where they are and what they're doing. Without a radio, you are effectively saying "Everyone else has to keep a specially watchful lookout for me because I can't be bothered to let anyone know where I am or what I'm doing."
Here's the news: My safety is my responsibility. Your safety is NOT my responsibility, but if you go radio-less, you make both our safety MY responsibility.
As others have said, when you have spent the $20K-$200K on an aircraft and then spent the money getting qualified to use it and then the money to maintain it, if you can't be bothered spending a bit extra on some kind of comms YOU SHOULDN'T BE IN THE AIR.