The proposal is based on a faulty premise
Yes, there's a general problem with misuse/overuse of comments. However, the suggested solution excludes legitimate and necessary uses of comments.
Comments actually have two primary use cases
That is the premise of this proposal and it is sort of technically true, but not in a way that supports the suggested solution. The existence of those two primary use cases does not mean that those are the only intended or appropriate use cases. As Thomas Owens' answer points out, that interpretation directly contradicts the guidance on the help/privileges/comment page, and excludes the intended purpose of adding relevant but minor or transient information to a post.
Excluded appropriate content and its impact
Restricting comments as suggested would encourage non-answers and low quality answers. Particularly on the computer-related sites, there is a variety of information that is appropriate to post, but not as an answer. A few examples:
- helpful links, especially if they do not contain an actual or complete answer
- diagnostic suggestions (unless the proposal here is to artificially shoehorn them into a request for clarification or suggested improvement with wording like "have you tried..." or "your question would be better if you include the result of XYZ test")
- speculative solutions that, if successful, would be only a lucky guess (You could argue that this should be posted as an answer. If it was, it might not qualify for deletion as "not an answer", but we explicitly discourage the posting of such low quality answers. If it turns out to be the solution, it can be expanded into a proper answer. Excluding them from comments would move the cleanup effort from comments to answers.)
- speculative edits, such as expansion of an answer that goes beyond the original author's scope but doesn't qualify as a stand-alone answer. The author could choose to add it, in which case the comment could subsequently be deleted.
- information by the post author that is useful but tangential or provides clarification needed by only a very limited audience, and would make the post excessively long or would be distracting if included.
Let me point out that some of the discussion I see about things that shouldn't be in comments ignores the definition of a comment as holding transient information. The argument goes that something suggested or raised in a comment should be acted upon and then the comment deleted. That is used as justification for the content not being in a comment at all. But that is precisely a contemplated use for transient information. Until the information in the comment is acted on, that is exactly where it should be.
The hole comments fill
This list is by no means comprehensive. We have standards and guidance for what makes a good question and a good answer. There is other stuff that is appropriate and important to post here in relation to questions and answers.
The current option is comments, which by default, tend to assume the definition of "anything that doesn't qualify as question or answer content". As such, a lot of stuff is posted in comments that shouldn't be posted at all, and yes, we could use a solution for that problem.
But I think overly restricting comments, especially contrary to our published definitions of their appropriate use, will do more harm than good, degrading question and answer quality by moving content there that is inappropriate or ill-suited for those posts.
Focusing for a moment just on content that is appropriate for posting (i.e., ignoring the chit chat, gossip, and similar stuff that doesn't belong at all), the vast majority of what I see in comments is in the best place for it.
Questions and answers are the site content and the focus should be on making those the highest possible quality. Comments exist to support the questions and answers. If there's going to be stuff posted that doesn't qualify as desirable question or answer content, and it inevitably will be, it's preferable that it be posted in comments rather than in questions and answers. Comment cleanup is much simpler than than the question and answer cleanup process.
Processes that are contrary to users' instincts
We also need to recognize that the Stack Exchange doesn't exist in a vacuum. The vast majority of posters are conditioned by their experience everywhere else. To the extent possible, the site structure should seem natural and easily understandable to users. We see inappropriate posting because that's what seems natural to a lot of users. I'm not saying we should turn SE into a forum or change our standards. Rather that the best compliance and the least cleanup will result from, where practical, doing things in a way that is consistent with what seems natural to users rather than contrary to their instincts.
If you look at the comment threads on other answers and at other Meta questions, there are also major differences of opinion on many things SE -- the appropriate nature of edits, whether certain content should be in a comment or an answer, etc. The SE philosophy is to purposely leave most such things loosely defined and subject to community culture. It provides the flexibility for users to work in a way that feels natural and comfortable within a general framework, rather than a rigid, one-size-fits-all collection of rules.
That approach fosters participation and contribution. Many people have strongly held opinions on how these things should work, but the issues will never be resolved, nor should they be; they aren't governed by singular rules that operate to the exclusion of all others. SE is successful because it accommodates this range of approaches and ideas, they aren't a problem to solve. That's why there's vanilla and chocolate ice cream.
Along those lines, I'll toss in a half-serious alternative that illustrates how to mitigate the problem in a way that is consistent with people's instincts: add a different type of comment. Reserve "regular" comments for content that is appropriate to be on the site (as defined on the help/privileges/comment page). Create a new type of post that is explicitly identified as a temporary, disposable post. Those would be automatically discarded after a short period (say 24 hours) unless the author of the post it is attached to (perhaps and/or a high-rep user), accepts it for conversion to a normal comment.
We might even allow users with <20 rep to post those as a harmless alternative to their posting non-answers just because they have the "need to post". Users >20 rep could use those for content that only needs to be temporary, including jokes, chit chat, etc., that they need to get out of their system. Of course inappropriate content would be subject to immediate deletion. We could even have filters to automatically delete ones that contain inappropriate language. It would cut way down on the cleanup required for both comments and answers.