I see the downvote button as serving at least one essential purpose: it sends bad content into obscurity quickly, where it won't be seen and won't generate further engagement. This matters becasue engagement on bad content tends to also be bad: arguments that are either vicious or go in circles, dogpiling, etc.

Have you ever seen threads that seem to never die on traditional forums, where people keep joining in an argument that goes nowhere becasue the thread is always visible, and the thread is always visible becasue people keep joining in? That's one of the major downsides of sorting by engagement. Reddit-likes not only don't do that, but with downvoting can do the opposite of that.

I've also observed that downvoting tends to be a substitute for making an angry reply - not just becasue it makes the content less visible, but also becasue it serves as an outlet for the human desire to to something to solve the "problem" that the bad content represents for the community.

Getting downvoted sucks, but I'd rather see that than a dogpile any day.