>>95969570
There were ups and downs but the inflection point for continuous descent began at Zendikar. Mythic rares lasted one (1) block before they became chase power cards. Frankly you could put the blame on Alara but I feel there they did briefly keep their promise.

Planeswalkers are also bad but Lorwyn was so kino overall I can't fault it too badly, especially when the Jacetice League hadn't taken over the story yet.