The whole premise to Back to the Future 2 doesn't make any sense.

One could argue that the further back in time one tries to fix a problem, the more likely "externalities" or unintended/undesired results will ripple or "butterfly" out from the intended change.

From that perspective, a quick jaunt into the future to change one small interaction between Marty Jr and Griff is the least threatening to the space/time continuum.

Trying to change it 30 years earlier is just too risky!

However, then one must realize 30 years earlier is exactly what happened in Movie 1, but none of that was exactly intentional, so perhaps that strengthens this argument.

But then one must also consider Movie 3. If 30 years is risky, what of 100 years?

Back to Movie 2- if 30 years can cause so many rippled externalities, then bringing 2 people forward 30 years- AND THEN RETURNING THEM TO THE PRESENT is probably a bad idea.

But the rest of Movie 2 and all of Movie 3 only prove that to be the case! So maybe one was right to argue all along.

despite being raised by the charismatic, energetic, brave never back down leader Marty Mcfly sr it seems jr was destined to be a awkward, dweeby pussy follower.

It seems no amount of raising would have changed jr's nature. And as the film says it sets off a chain reaction that destroys the family. Even if he did warn his son about this specific event and try and raise him better and he backed down. its likely his son would have gotten drawn into another one of Biffs plans.

this way his bad influence goes to jail, Marty doesn't give into being goaded ect.