I think it's moreso that the change to Zed and Fiora is about balancing them better.
The change on its own is terrible. ADCs NEED a counter to Zed's ultimate.
The problem Riot is seeing, at least I think, is that while Zed feels insanely strong, his power drops off too hard as soon as QSS is obtained because his ultimate is ONLY useful against a squishy target. So they think he's balanced, but it isn't healthy. They want to make it that he can still get a good combo off on his intended target, so that they can nerf him elsewhere.
On WW/Malz, they can be equally effective suppressing a tank. The whole team won't build QSS. Stopping a tank from CC'ing your team or diving on your back line or peeling for his back line is pretty effective too. E.g. Malz can suppress the enemy Maokai at the start of the fight so his allied Wukong can dive the enemy ADC more easily. As a side note - both Malz and WW are already countered by the tank being able to CC them to break the suppression, so suppressing the tank actually SHOULD be the optimal strategy. It will just take a different mindset, you're no longer playing these champions to cheese a squishy, you're doing it for insane teamfight power with coordinated players.
Suppression is also already stronger than most CC because Mikhael's can't work on it. So each player needs to build QSS, but only 1 or 2 really will. As opposed to say, the support building Mikhael's to counter a Lissandra ultimate.