Might have been planned obsolescence, but the reason Intel Core Duo was not supported by Lion was due to Finder being exclusively 64-bit, and thus the OS going 64-bit only. Only the early 2006 black MacBook was affected by this, as all the later ones used Intel Core 2 Duo.

That means the breaking update came five years after the release of the computer, but Snow Leopard still had another two years of security updates (AFAIK). Personally, I don't think that's unreasonable.