The only scenario I could imagine where the OGL could ever hurt Wizards would require a perfect storm of rather unlikely events: (a) If WotC released a buggy, unwieldy, and closed 4th edition that somehow crippled popular aspects of the game -- no elves or wizards or dungeons, but lots of psionic-wielding vampiric tinker gnomes with flintlocks in a gritty planar city setting; (b) and if, in response, several big d20 publishers banded together to publish a "free alliance fantasy RPG" that substantially improved, expanded, and codified the 3rd edition ruleset; then (c) maybe, just maybe, enough people would go with this free alliance game that WotC saw a hit in sales. And even then, I don't think it would be a substantial hit. Folks will follow the D&D brand even into Mordor, if only to complete their collections.
But I really couldn't see WotC being so short-sighted and ham-handed to badly botch 4th edition. And sadly, I really couldn't see enough cooperation between d20 publishers to create a serious alternative.