In reply to Jeff Chandler.

Other projects have moved toward separating framework and application layers to solve this problem, but for all practical purposes that is still like a fork where now you have to support two projects and products.

Another approach would be to give the legacy product a generous EOL and a really solid upgrade/migration tool. With a good marketing campaign and community buy-in -- especially from hosting companies and others positioned to offer migration support services -- the majority of end users could be moved from old to new WP. Failing to achieve that would be a disaster, but it is one that has been seen before -- lots of opportunity to learn from others' mistakes.