I too think negative signaling is valuable, but I doubt Twitter will ever correctly implement that. This is evident from the state of "report tweet" menu. Reporting can be thought as a strong negative signal that happens to be also ToS violation, but its implementation is far from working; not just political figures, but I've seen even direct death threats against minorities with numerous reports ignored multiple times. Twitter seems to have no incentive to fix reporting and I don't think downvotes will be different.