Question. What the hell can you do against BJ Penn even if you manage to get him down on the ground, assuming his ribs & everything else stays in place? BJ probably has the best defence on the ground and it's going to be very hard to get anything done on the ground against him. He had GSP neutralized on the ground, GSP got him down but he got all tied up and couldn't pass to a good position or get any real damage done.
I could also point out the example of the 2nd Hughes/Penn fight. Matt is one of the more dominant ground & pound guys and look what happened to him before BJ popped his own rib. Hughes got tied up on the ground and almost got submitted.
You just ain't gonna look good on the ground against BJ Penn, simple as that.