There are additional reasons, the existing moderator tools would not be sufficient to deal with the kind of abuse that would be possible if anyone could post comments, there is simply no meaningful review of new comments on old posts.
And acquiring 50 reputation is generally not hard, that is 10 upvotes on questions, 5 upvotes on answers or 25 suggested edits that improve other posts. You could also earn 200 reputation on any other Stack Exchange site and be able to comment on all of them.
All of a sudden I find myself having something of value to add, and I want to post a comment, and it says I can't. So I have to wonder, how do I even get rep if it won't let me add value?
Why not let EVERYONE comment but it hides comments from people with low rep by default. Anyone can view those comments, but people with enough rep can also add points to us low-reppers who just wanted to add a comment.
Just a thought.
2. Once you gain some reputation, you will understand why `points` are an important factor to use functionality. It is a good step.
3. Not to forget, since the quality of the question is degrading it also means that it is easy to answer questions on these days since the questions are asked in a different way but usually have same answer.
The people who have the most influence on SO are not the ones most interested in using it for questions, but the ones most interested in improving their "score". It should be obvious that this outcome is both perverse and extremely difficult to avoid.
Sorta like HN.