Maybe.
These kinds of arguments are dumb because you can literally speculate about anyone and make up any scenario. Could Robbie Lawler have been the best if he trained at a different camp?
Could Paulo Costa been the best if he wasn't too playing goofing around and playing games? Could Anderson have retired perfect in the UFC if he worked harder and not gotten lackadaisical?
But since we're hypotheticalling (yes, that's not a word)
It's possible. Brock would have needed to work on his striking at a much earlier age, learn submission grappling, not been struck with diverticulitis, and started training when he was much younger. Also (and I don't know if you can teach this), become comfortable with getting hit, which Brock was very much not.
If all of those factors were in place, he could have been the HW GOAT.
Of course, that doesn't factor in the (obvious) role roids played in his whole career, but that's for a different thread....