Why did they change the Keen rule and not the property?

[Kae'Yoss]

If it's not to fix something broken, then, it doesn't belong in a revision, it belongs in a new edition. I know I'm not alone in THAT opinion -- Monte Cook shares it, according to his own review of 3.5e ...

We didn't say you were alone with that opinion. But it isn't the only one, either, and Monte's opinion is just that: another opinion. While his has more weight than that of your average gamer, since he was actively involved in creating D&D 3 (and his name is still there on the books, even in 3.5), Andy Collins's opinion doesn't weigh less, since he was involved in creating it, too.

Personally, I think it was OK to change that, for it could get out of hand easily. Sure, the real problems aren't in the Core Rules (although I don't think it is a "critical" hit anymore if it can happen almost with every second roll), they start when you take supplementary material and third-party books (as the weapon master PrC and some "Improved Improved Critical" feat from Quintessential Powergamer or something), but I think the rules should provide a solid base that can't be overcome so easily by 3rd-party material, especially since the whole concept of D&D/d20 heavily depends on third-party material.