You are missing the point - debian/source/format allows you to make it explicit that the source format is 1.0, if you want to stick with that.
You are missing the point - the planned switch of the default format isn't considered a good approach by quite some people. If something new gets introduced, switching to it should be done in wake manner. Other tools do that too, so why can't a central package as dpkg use that approach? I especially want to mention that a point release update for stable was required to get it working in stable like it should.