I'm honestly not sure they are trying to prevent "alternate PHBs" as much as protect against the following.
1) Gross republishing of the SRD rules by themselves, either for profit or for free. The free SRDs can be used as a replacement for their core products. Their core products sell more than all their supplements combined. Regardless of their use to the community, it would make sense for them to close up the license.
2) Right to revoke. I think Orcus might agree that licenses involved a contractual agreement between two parties. Most regular contracts have an exit clause--"tenant at will" for instance means the landlord and tenant can both decide to end the contract from month to month. The OGL has no built-in expiration date or any sort of revocation clause. This is great for the licensees, but not so great for the licensor. From a business standpoint, a perpetual license that doesn't allow any sort of revocation for standards, content, morals, etc., is not a good one from the creator's standpoint. Now, a lot of third-parties and fans don't like it because they're afraid Wizards could shut them down "just because they are too good", so I understand it's risky.
So those are the two areas I think they're trying to prevent from happening.