This move has nothing to do with usability or device capabilities or software support. The only reason for turning them off is to remove a hole in their DRM. These old devices allowed you to remove the copy protection from their books (hardly anyone else uses DRM these days anymore). Removing the old devices makes freeing your books from DRM just much harder.
I often laugh (cry) at the Kindle Product Manager team who ship nothing but DRM updates.
How about a dictionary modal where the font is the same size as the page text..? Hard to imagine what they do all day, given they do seem to force updates but nothing seems to improve