While downvoting is perceived as a flag and rightly so as it is refereed to by the system as such, and constantly refereed by the community as downvote, it will garner all the negative attention associated to flagging, and act as a confusion and misunderstanding primer. It's kinda a flag but it acts as a downvote with the power to also hide content, not just decrease rewards based on VP and SP. Downvote and flagging should be two different systems, flagging is for abuse and flag abuse, and that should be equal in power across the board, and the current flagging should just share a place next to the upvote as a downvote and it should be much more draining on voting power, as a means to balance it out and de-incentivize abuse and then remove it's ability to hide posts and give the new flagging function that ability and also to negatively affect the reputation, as well as allow for flags themselves to be flagged, and if there is a consensus of 2 or more affect the reputation of the flagger, as to stop sybil attacks with flags peg the flagging ability to a good reputation score that can be achieved by new users, maybe 50ish.

As for downvoting being used for purely negative posts or spam it's clear that those parameters have changed considerably as it is explicit in the very first reason it lists when you hit the flag/downvote button and I quote:

Flagging a post can remove rewards and make this material less visible. Some common reasons to flag:

1. Disagreement on rewards