Don't forget that SSL/TLS normally resists downgrade attacks -- but browsers use insecure fallback due to servers that don't properly handle the SSL handshake. The whole purpose of TLS_FALLBACK_SCSV is just to re-enable the TLS downgrade resistence while retaining compatibility to those broken servers.

I wonder what would effect more users - disabling SSLv3, or disabling insecure fallback?

what if browsers support SSLv3 but throw a giant warning: "INSECURE SITE"? The message has to be framed correctly of course.

I think the real benefit of that sort of solution would be to put the blame squarely upon the shoulders of the outdated sites in question without the browser vendors worrying about losing customers (A win/win?). It's an opportunity to inform users about the major security flaws on sites like Citibank and put some pressure on them to take this sort of security flaw seriously.