I think the line between "pure" and "mixed" abs, and "good" and "bad" Abs is not the same thing. Seriously excellent ABs may have been mixed yesterday, while really shitty ones might have been pure for 40 + years.
If anything I think this would be the case, real working dogs simply aren't kept pure. If there's a correlation it would be with purer ABs (crossed ab to ab for longer) tending to be worse and further from what ABs are supposed to be than mongrel mixed up ABs with pitbulls and catahoulas and bullmastiffs in their recent history.
Starting from scratch is a different thing again though, you can still produce great gripping dogs no doubt, but calling them american bulldogs without a shred of american bulldog history in them would seem a bit suspect. Calling a strain of american bulldog with lots of outside influences repeatedly through history and continuing in the present day, american bulldogs, OTOH, is perfectly fine, and in fact anything less, being kept "pure", is what I would argue is actually turning into something other than a real american bulldog. Because they, like everything, are supposed to be mutts. So continueing with mutt-style breeding would be the correct way to keep them "pure" american bulldogs.