Giving proper credit to others is IMHO a good practice precisely because this is not a competition arena. IMHO this is just fair play. Of course, a comment does not reserve the right to answer a question. However, I personally feel that if there is a comment which the answer I am going to write has overlap with, it does make sense to acknowledge the comment. And, as Johannes_B is saying, asking someone who answered a question in a comment to convert the comment into an answer is IMHO a good practice, I cannot see anything wrong with it. On other sites like this one sometimes even moderators convert comments into answers. Why is there anything wrong if someone asks you to convert your comment into an answer? Isn't that just fair play, and nice to see that others read your comment, acknowledge it, and ask you to make it an answer? Isn't that precisely the opposite of competitive behavior?
Is TeX-SE a competition arena?
This depends on how you view things. The stackexchange websites have some built-in competition just by the fact that there is a reputation score, and that you can "rank" users according to their reputation. So if you take this score and these rankings seriously, then there is a competitive element in our main site. The fact that there is a sportsmanship badge may be interpreted as the intention of the designers of our site to encourage a fair competition. What one can read into some user with a high reputation score and many answers not having gotten this badge I am not entitled to say in public but one could argue that might correlate with a certain mind set which is against the the idea of fair interactions between the users. Of course, I do not know what the true purpose is of the reputation score, but it might be an incentive to provide novel questions and working, timely answers that work and solve problems. And yes, one can find statements that "someone beat me by 3 seconds" and the like. The slightly later user may (a) see no point in posting an answer with substantial overlap to the earlier answer and (b) be somewhat disappointed and try to speed up when attempting to answer the next question. You can call this competition. And this competition does have draw backs. For instance, I do see a point that this kind of competition makes us entirely focus on the new questions because everyone wants to be first. And I do agree that this is not necessarily a good thing. This score does make users go for the most "lucrative" posts, and it seems that the newer the more "lucrative". If you have a concrete proposal how to replace that (IMHO stupid) reputation scheme by something that adds incentive to write useful, clear and timely answers from which all of us benefit, I am all ears. Yet I fail to see how anything you mention will help us to improve on this. It may be that I misinterpret your statements, though.
Then there is the question under which circumstances it makes sense to post an additional answer, where the opinions mentioned in the answers agree to some extent. I do not want to repeat my view here, but, yes, I would call it picking if you repeat an answer and the only novelty is that one saved, say, two keystrokes, and find it even somewhat inappropriate if one does not honorably mention the answer that one has shortened. If you have a very different opinion, you may want to explain it there as well. (I cannot resist mentioning that one user writing an answer left, which seemed to have played a role in CarLaTeX's decision to write this post. Maybe we should have taken their opinion more seriously. And please do not get me wrong, but there might be users who feel that those who write three answers to a question are those who are particularly keen on harvesting reputation and/or to "steal" the tick from another answer. I personally am fine with posting several answers but personally try to just write one in which I either add two options, or select the one that is most suitable. So I definitely do not want to criticize for posting three answers, but would kindly like to ask you not to criticize others for doing things that you prefer not to do.)
Competition and academic honesty
Related to this is the question of academic honesty. (Interestingly, there another user who sadly decided to leave our site made a contribution. Maybe we should have taken them more seriously, too.) The upshot seems to be that, even though academic honesty is not enforced, several users feel that it is something good which precisely helps us to have a friendly atmosphere. Of course, there is the possibility that those who feel that giving proper credit to others is unnecessary just didn't want or dare to write an answer. Therefore it might be useful if you could add an answer to this question that explains why that view is appropriate. I personally feel it just fair play to mention earlier posts with which newer answer overlaps, and also to "cite" related answers to different questions if they develop. Why? Because the author of the post I am borrowing from/overlapping with will in most cases like it, and it also prevents us from writing the essentially same answer to the same question over and over. After all, there is no mechanism to weed out duplicate answers in the same way we close duplicate questions. (I also cannot refrain from remarking that when it comes to new posts by others, you seem to share the opinion that older answers doing similar things are to be mentioned, so I really have a hard time understanding why you do not want to apply these standards to your own posts.)
What is the current status and how can we improve the climate?
I am not experienced enough to judge whether or not our site is in a decline. Nor do I know for sure whether the decision of two IMHO very good users, whom I really miss, to leave is related to this (but since you were mentioning this, shouldn't you be concerned that they left just after you became active again? ;-), but I did see them complain about academic dishonesty, too. In any case, I do feel that giving proper credit to others is something that does help users to coexist in a good atmosphere. Apart from this, I do believe that most of us here have good intentions and a lot of tensions come from miscommunications, some of which are related to the fact that a substantial fraction here, including me, are not native English speakers. This is something that IMHO we should try to keep in the back of our minds when interpreting statements by others.
And if you are really desperate, you can ask TikZ for advice.
\documentclass[tikz,border=3.14mm]{standalone}
\usepackage{tikzlings}
\begin{document}
\tikz{I can't \bear this any more, what should I do?}
\end{document}
subcaption
are imho different here: they are code-copying-and-editing, but "stealing" comments is idea-stolen. Many times I didn't understand the whole code posted, but I could still answer, because I did not get the idea, but I got how to copy-and-edit the code. From your point of view, if the commenter doesn't trying to post an answer but only a hint, then it is ok to change that hint to a complete answer, but who knows the commenter is typing the answer or not?