The majority of AP users' primary goal is to burst a target down, and shield reduction matches that much better than healing reduction.
There are very few instances of mitigating the effects of shields in the game and of the two that currently exist, they essentially remove the mechanic (removing the majority of effects that coincide with them while they persist) instead of mitigating its effects. Removing part of a champion's kit via a gimmick isn't cool.
Changing it to shield reduction would give it a very clear goal, which is bursting a squishy target down, which often you cannot do any time shields are decent. If you FAIL to burst someone down with the current Morello's, rather than allowing them to have a fighting chance via sustain, which they should via basic game design, you cut their healing and almost ensure a kill unless you didn't deserve to kill them in the first place.
Yes, I realize that healing and shielding accomplish very similar things in the thick of a fight, with the main distinction being that shields are about preventing damage while healing undoes damage.
If someone is to have an additional gateway to killing someone through shields, it should be the burst champions. If someone is to have a gateway to killing someone through healing, it should be the dps/attrition champions, via Mortal Reminder.
Burst is an answer to sustain by default/design and doesn't need further help. In the cases they fail to burst, they shouldn't be rewarded further.
######Bramblevest still shouldn't exist as a GW option because it doesn't match the goals of the tank class and only lets them stat-check. If tanks are to be viable, specifically top lane, it should be by not sucking complete ass, not by being able to cheese early game vs some match-ups by stat-checking specifically with this item. This is further emphasized by how much its completed item, Thornmail, sucks.