If he runs again no matter what happens, he will lose. I can safely assume a Con D to succeed him. You also got to keep in mind how diverse TX will be from this point on, that is something that has been overlooked in this conversation.
I think it being a midterm year makes it a toss-up. If Texas dems can find an old white boring conservative male democrat with legislative experience, they should have a good chance of winning.
Chet Edwards is the only hope I guess under your definition. But I don't know people in suburban Dallas and Houston would like that though, they hate Con D's.