I think they’d rather just keep the system consistent and avoid confusion over why you can roll on one thing but not another. If you have exact same item on the same or higher track, you can’t roll.
I can absolutely see cases where champ track items are better than hero track ones, especially early on when people are less willing to spend gilded on hero track to actually make them any better than champ.
And for things with lots of stats or unique effects I can absolutely see more than a 13 ilvl downgrade still yielding a DPS increase.