Having the legacy template (Blue Eagle) for a while does actually make a lot of sense. One of my template's is mostly J3 compatible but has some missing CSS code that makes the new template not render right. As a result I am temporarily using Blue Eagle until the fixes are made on my end. So what I recommend doing in the future with major releases is adding "(legacy)" or "(deprecated)" to the old template title. The W3C does this with HTML where some tags are deprecated but not quite obsolete. This status makes it known that it can be used, but is no longer supported. Just like how K4 can be used with J 2.5 but is not supported.
I suppose then in the future you either need to decide whether to support a legacy template short term (3-6 months) or with major updates mention that only the new template will be supported. I am very grateful for the new template and much prefer support for the new one. But I also understand the frustrations of those who use K4 and have not make the needed steps to make their site template work with the new template, forcing them to stick with the old one for now.
