I suspect that part of the issue is that people want to be able to do things that you used to be able to do on hardware that used to support it. Unfortunately, that can be difficult to accomplish if old installers are taken down or if incompatibilities are introduced. Another issue is that some people are willing to make due with the slightly lower performance incurred by older hardware. Keep in mind that we're talking about buying, managing, and reading ebooks on Pentium IV's or later here -- not HD video production on similarly aged products.

I've said this before, and I'll say it again: obsolescence is largely in the perspective of the user. Some people are happy at the trailing edge. Others are satisfied with nothing less than the latest and greatest. In some cases it makes sense for businesses to draw that line, such as Microsoft deriving most of it's revenues from Windows from new hardware sales so they need to produce software which utilizes that hardware. In other cases, it makes no sense at all for the vendor to draw that line. Ebook sales, management, and reading does not rely upon hefty hardware so it makes relatively little sense for Kobo to dictate the latest and greatest.