For weakness of Inoue to unfold, he just needs to fight a better boxer than he is. He may be dominated the bantamweight division, but I do not think that he will do the same in the super bantamweight. Super Bantamweight is a different division and there are several known boxers that can possibly defeat Inoue in this division. The boxer in this division have heavier punches and more resistance to blows or punches, and since Inoue is taking a shortcut, he probably doesn't know the extent of the punching power of this division.
I very much agree, however, this is what the sport of boxing is supposed to be. The best boxers should challenge the other best boxers in another weight division or in their present weight division. This will bring the sport forward again and not need to become a comedy show to get more attention similar to Jake Paul. Fulton vs. Inoue, Tank vs. King Ry and Haney vs. Lomanchenko are good fights for the fans and the sport.
But even in the 140 lbs, we have seen Josh Taylor unify the belts and then 135 as you have said, Tank vs Ryan Garcia, and then we have Haney vs Loma. And as if this lower division are mocking the supposedly best weight class because it now becomes a joke, Spence vs Thurman fighting at 154 lbs and with no belts on the line.
The welterweight and super-welterweight is a known division that is full of vicious boxers all over the world but now, that's not the case anymore because Spence Jr. and Al Haymon is giving the division a bad reputation because they are taking a full advantage of their influence towards the governing bodies to make a mandatory fight without risking the belts of the champion.
That is why the avid fans are now taking more attention from the lower weight class because aside from its exciting fights that is produced every couple of months, we know that the fights taking place at the mentioned division is fair and square, so different from the upper weight class these days.
Anyhow, May it is, Fulton will be ready by that time, but it could be very different as it's hard to get a sparring partner similar to Inoue and probably the most important is the power of Naoya. Because it's not going to be replicated by any training partner in the gym.