Thank you for your post and your concern, and for backing it up with some concrete evidence that we can look at! I admire and appreciate your desire for a healthy site, and I think you raise some good points.
On the general concern: Taking into account all the pluses and minuses, I don't think the site is being too harsh, by and large. No policy is perfect; any decision rule will have false positives and false negatives. The current policy is one that was able to gain community consensus, and my impression is the number of questions that are wrongly closed is vastly outweighed by the number of questions that are properly closed. The question is, if not the current policy, then what else, and will it be any better? I think anything else is also likely to make errors, too. So showing one example error is not, to me, a fully persuasive argument for changing a general policy.
This specific question: I think it is debatable what should be done with this particular question, and there are reasonable arguments on both sides. My personal opinion is that it was right to close the question, but the wrong close reason was used.
I believe this question should not be closed as a problem statement question.
I do think it should be closed as "too broad", as it basically asks four questions: (a) given a distance metric in a vector space, can I define a norm? (b) given a distance metric in a vector space, can I define an inner product? (c) given a distance in a metric space (not necessarily a vector space), can I define a norm? (d) given a distance in a metric space (not necessarily a vector space), can I define an inner product? It's usually not a good practice to ask four questions in one post.
Is it a good question? Some of those questions are not great ones. It looks to me like question (a) is answered by Not every metric is induced from a norm, and thus I would vote to close it as a duplicate of that question, if it were posted separately. Also, Wikipedia gives some hints about question (a). I think it's reasonable to expect people to have done enough research that they find information that already exists in standard resources like Wikipedia, other questions on this site, basic textbooks, etc., before asking. I don't think we're doing the world any great service by repeating material that already exists. Once (a) is answered, the same counterexample is going to help a lot with the other questions as well.
So in this case, I think the proper remedy would be vote-to-close as too broad; if the question had asked only (a), I would vote-to-close as a duplicate; and I think the best thing would be for the original poster to go through that material and determine if there is still an interesting question that hasn't already been answered on this site or in standard resources.
About homework and PSQs: The current policy doesn't care whether a question is from a homework or not; that is irrelevant. What matters is whether it is a "problem statement question". The policy applies to all PSQs, regardless of whether they come from homework or not. These questions cause more or less the same harms to the site, regardless of whether the question came from a homework assignment or not.
To avoid becoming rigid and dogmatic, I think it's helpful to recall why we have a policy about PSQs, and then in specific cases, query whether those reasons apply to that specific case. Some common elements of PSQs are: They are often super-specific and unlikely to be helpful to anyone else in the future (unless someone else happens to be facing exactly the same exercise); it's debatable whether they are helpful to the question-asker (if we are robbing them of the opportunity to practice, we may not actually be helping them); there is a massive supply people wanting to post PSQs, more than we can keep up with; a very large percentage of PSQs are bad for the health of the site; they often don't contribute to the mission of the site, to build an archive of knowledge that will be helpful to others in the future; they risk driving away some important contributors to the site. So that gives some alternate criteria you can use to evaluate a question to determine whether any particular question should be treated as an exceptional case.
If/when you see a PSQ that avoids all of those common flaws, I think it's great to edit it to improve it, vote to re-open it, and/or advocate for re-opening it. I don't think policies should be treated as rigid rules that can never be overruled. There probably will be exceptions, and I agree with you that dogmatism is unhelpful.
About valuable questions that you see as wrongly closed: When you see a question that was closed but you think should not be closed, I encourage you to edit the question to improve it. Almost always, there are opportunities to improve the question. I think editing is a constructive path that benefits the site, and is a show of good faith towards improving the site and advancing the mission. Debating policy doesn't always lead somewhere good. In this case, I perceive a lot of debate about whether this question should be closed or not but I haven't noticed anyone stepping up to edit the post to address its limitations and improve it.