mathanxiety good question. Perhaps one of the Mrs proponents could answer that one. It's an interesting question because those in favour of choosing Mrs have repeatedly said that they wish to have that choice - that choice would no longer exist if everyone was a Mrs. Just as no choice exists for men currently. So there would in fact be less choice overall. Not that that is necessarily a bad thing.
Incidentally, I find those who are against Ms simply because they don't like the sound of it a bit strange if I'm completely honest. Surely it shouldn't be about the sound the word itself but what it connotes about the person and the bigger picture. And talking of choice, if a man doesn't like the sound of the word Mr, he doesn't have a lot of other choices, does he? 
No one so far has said that they disagree with everyone being a Mrs, so perhaps we do have agreement? However, I do remember someone earlier in the thread saying that they are happy to be an (adult) Miss and wouldn't want to change.
bzzbee I too wonder about the future of surnames. There doesn't seem to be any easy answer to that one. Other than a couple choosing their own new surname when they marry and/or have children. Some kind of combination of their existing ones or something new completely.