IMO, a post shouldn't get less visibility just because I disagree with it, if it has a good point, or is a good stepping stone for discussion.
Agreed, however the history of reddit and this platform have shown that even threads with significant levels of disagreement remain visible with valid conversation and a well-written OP.
Can be accomplished with upvotes alone.
Without downvotes to balance the upvotes, threads would not reflect dissenting opinions at first glance. What if someone makes an aggressive and unsubstantiated thread and it has 500 people that are opposed but 40 people that upvote the thread? Those 500 people's opinions are completely ignored.
Yes, comments can handle this sort of thing, but there are many situations in which commenting is completely unnecessary or not feasible as a replacement to voting.
The report button should encompass this function.
It could, but the whole point of allowing the players to regulate themselves is that it takes some of the load off Riot. It's also significantly slower to wait for Riot to reach the report in their queue than to mass downvote a comment.
Which can be done via comments, and upvotes on those comments.
We have been over this; commenting is not always feasible or even worth doing. What you are suggesting is essentially replacing downvotes with a comment saying "I don't agree" or "this is clearly spam, why am I being restricted like this".
Removing the downvote option doesn't change the accessibility of options at all, with the exception that you now have to comment do express your disagreement. This is a good thing, IMO.
It is extremely restrictive and actually creates a massive imbalance between positive and negative feedback. If you can show your appreciation with a single click, why can't you show your lack of appreciation with a single click? While we're at it we might as well disable all voting and just return to allowing players to constantly bump threads with spam, because without the ability to counterbalance positivity the system is totally inaccurate.
Here's the thing; the concept of disallowing downvoting is innately assuming that a significant portion of the community will abuse it; you are essentially expecting the worst out of everyone. It's great to have a system that stops this from happening, and realistically speaking voting systems do work to accomplish that. The general population will use voting correctly, some people will occasionally misuse it, and the ones that consistently misuse it will probably get banned or drowned out by everyone else in the community.