One option open to SEGA is to charge for DLC as planned, and release the SDK. But only make third party content usable if ALL DLC packages are installed. It's not too much to expect, really, because DLC will most likely include extra assets anyway. Stuff like models, textures, sounds etc.
So, to my way of thinking, sure, charge for DLC and make the SDK freely available to modders but any content generated will require that DLC be installed. That way, any fan made content is still dependent on sales of DLC and we still have a modding community. I've got no problem with that.
My problem with the DLC model as it stands is that it's sole purpose is to rake more money out of the fans without any great return; but adding SDK support onto that lessens the pain we have had to endure to date and everyone benefits from a modding community. It's not too late to have one despite the game's poor reception. Otherwise, I just don't think we will see one beyond texture and sound replacements without an SDK.
-Windebieste.