In my opinion if you disagree don't upvote, or at least downvote until 0. So people know the overall sentiment is negative, but there's not need to show how negative it is. If it's so bad to be offesive or innacurate, then report it. But downvoting until it's negative feels really overkill and feels like at the moment everyone not only disagrees, but hates you.
The constructive replies you talk about depends on the subreddit. There are subreddits that are literally lifesavers. For example subreddits to overcome emotional abuse and abusive relationships, you'll find support there that is simply amazing and not abusive at all. Or subreddits about certain niches that people are passionate about.
Unfortunately toxic people are sadistic and get a kick out of abusing people, so it's bound to happen. Best way is to report it, so the moderators either warn the person or ban them.
For example, that video by The Verge where that guy builds a PC in the stupidest way - it's got thousands of likes for some reason but 10s of thousands of dislikes because it's not the way you should build a PC
Now the dislike counter is gone, those who see the video think "oh this has thousands of likes. It must be good".
Downvoting does attract idiots and pile-ons, but it does serve a purpose as a a lose indicator of content/comment quality
One area that seems inevitably would have to change is Reddit’s linking to pirated video clips. I’m sure the clips won’t go away but would be altered in a way so that reddit has deals with copyright owners and unskippable ads are played.
Ultimately all these media funded platforms under-weight advertising to begin with to build a user-base so become worse to use once established.
Rising engagement could also come from content that got linked and from people just visiting the page.
Buying votes for reddit is allegedly quite cheap as well.
I also feel like all social media the there are very toxic aspects to Reddit. I used to be very active on there, but after receiving constant abuse and very few constructive replies I now rarely participate in discussions there. Again with the company going public like Facebook and Twitter, Reddit could come under more scrutiny for this.
Assuming you buy for a fair valuation I don't think it's a bad investment per say, but I think there are a lot of risks there. I find myself wishing for an alternative to Reddit a lot these days because I really miss the high quality discussions I used to have there 10 or so years ago.