My understanding was that the developers suggested that if the complete new SDK code was available, then they would consider porting to the new version. Except if they believe that the current stage of development makes it too late for such a conversion.
At this point, if (this is entirely hypothetical and I mean to start no flame here -and I know that the devs have no intention/willingness of doing this) Black Mesa went commercial, they would get the code that Dear Esther have. But then, they would possibly have to deal with new bugs and issues from porting to the new engine.
So, since the SDK is ready for 3rd party developers to use, why is it available only for commercial games and not for mods? Is it an extra incentive for buying a license for the Source engine?