Well, to be fair, all they're doing with Gurren Lagann is compiling the plot into the space of two films. Sure, it's milking, but that much isn't new, and I'll take it over "yeah, we're making more even though we ended it as well as we could." You're right, they can't make a long franchise out of this. The best that they can do with it is write a good epilogue and be done with it. The heart of the story has been told. Well, they could also do a prequel I suppose, but they never seem to make those.