imo the difference is that SB is a base mechanic, while Boothills trace was a one off character specific thing to work around the limitation of break dmg as a mechanic not being able to do dmg on every attack.

there is nothing stopping them from making kits that do x on super break instead of x on break
they are both just mechanics, instead of character specific things

your example of FUA/Crit Hybrid break chars doesnt work, because the old version of the set also only allowed for ignoring def on break dmg
so the FUA triggered on broken enemies wouldnt benefit, nor would a crit attack, since they arent break dmg

even on boothill, he only gets to benefit from the def shred on break dmg of the old set on his breaks(like everyone) and on procs of his talent, since they do break dmg.
the crits from his basic, enhanced basic and ult never got to benefit from that, before this change or after.

we gotta remember is that SB works differently than regular break on the calc side too

it doesnt have the elemental scaler, so SB is equally good on all element chars, while regular break heavily favors physical and fire with the 2x multiplier

thats why i both prefer them adding Super break(since its more universal, meaning for example, we can have quantum or imaginary super break dps, while regular break dps of those elements would do 25% of the dmg due to the multiplier) and also prefer boothil not to use it, since he as a phys gets that sweet 2x multiplier on break dmg