The league had a little more than half the teams when JB played which means LT is the one playing against watered down talent. One thing that is often overlooked is that in the 1950's and 60's football was king.. if you were a good athlete you played football; sports specialization the way it exists today was not around then. Imagine an NFL today where there were only 15-20 teams, so the bottom 40 percent of players were eliminated, and most of the Shaq's and LeBron's were playing football... then LT would be playing against the same relative level of competition JB did.
I'm not saying that athletes aren't better now-- clearly they are. But if Jim Brown had access to all the training, supplementation, etc. nowadays I'm sure he'd be just as dominant. Jim Brown was so obviously superior to the competition throughout his whole career, that LT would have to have a few more years like this in order to be considered the best ever.. although IMO he has to be top 6 or 7 based on what i've seen thus far