One problem I immediately see with this is that it's difficult to define what a serial troll (excluding bot accounts) is vs someone a community doesn't like for

One problem I immediately see with this is that it's difficult to define what a serial troll (excluding bot accounts) is vs someone a community doesn't like for whatever reason, using a downvoting system at least provides some leeway in discussion that is on the boundary of what is and isn't commonly accepted for a given comment at a given time within a conversation.