Wasn't that basically the intention behind the Upvote and Downvote systems in Lemmy, StackExchange/Overflow, Reddit, or old YouTube? The idea being that helpful, constructive comments would get pushed to the top, whereas unhelpful or spam comments get pushed to the bottom (and automatically hidden).
It's just that it didn't really work out quite the same way in practice due to botting, people gaming the votes, or the votes not being used as expected.
Yep the flaw is assuming that humans would actually select for constructive comments. It's a case where humans claim that's what they want, but human actions do not reflect this. We'd eventually build yet another 'algorithm that picks what immediately appeals to most users' rather than 'constructive'. You'd also see the algorithm splinter along ideological lines as people tend to view even constructive comments from ideologies they disagree with unfavorably
Bots on Reddit already steal parts of upvoted comments and post them elsewhere in the same post to get upvotes themselves (so the account can be used for spam later)
Even with context they can be very difficult to spot sometimes.
Sometimes you might need an urgent answer (eg, overflowing sink or a weird smell coming from an appliance problem) and don't have time to fill out a serious form