@Ryan - I think a supported legacy branch -- with upgrades to new versions every eighteen months or so -- would be useful to a LOT more than 0.1% of WordPress users. In the earlier days of the project, most of the users actively followed it and were eager for the new features. As the WordPress community grows, many of the new users are less technically savvy and less interested in the software for its own sake -- they just want to write about their passions or update information about their businesses.

Even as a somewhat technically-savvy user, I get tired of upgrading all my web sites every three or four months. Something inevitably breaks in a plugin or in the theme. Spending an hour or two upgrading and repairing each web site adds up to quite a bit of time I spend messing with software instead of writing. I would much prefer to have to do this once every eighteen months and not every four months.

One other important question related to your 99.9% number: how many more people would become WordPress users if a particular branch were supported for a longer period of time? I have talked with web developers and designers in my local area (Dallas) who have stayed away from WordPress because of its bad reputation for upgrade fatigue.