Forums that are known for rudeness will primarily attract and retain people who are not as affected by being the targets of rudeness. In other words, it eliminates some subset of the people who could participate. The ruder and more unkind the forum, the more people it fails to attract.
My own belief is that kindness is desirable in and of itself, and that one is not morally superior just because one is less affected by unkindness. Even from a purely selfish perspective, it feels better to be kind than to be unkind. It is clearly possible to have robust, fact-based, rigorous discussions of technical subjects while still being both kind and polite, and I think that these discussions are just better than ones with the same level of rigor but added unkindness.
While it is certainly impossible to never hurt anyone’s feelings, we can at least stop trying to and we can be open to change when our actions don’t have the intended effect.
However, to convince somebody of something, I should start from premises that are presumably shared, rather than ones that may not be. Assuming that one wants Haskell forums to grow (which is a reasonable assumption, as programming languages become better when they’re more popular due to network effects), we should want more people to feel like they can participate. This helps us all. Assuming that one wants technical discussions that lead to the best outcome, rudeness is also maladaptive. If technical discussions are to be “won” by hurting the other participants until they withdraw, then we’ll end up with worse outcomes. If someone seems to be repeating themselves and ignoring an argument, it’s usually useful from the perspective of getting a good argument to ask for clarification on the assumption that there’s been a miscommunication, rather than to flame them until they withdraw. It’s not like plain-text fora inhabited by people with many different backgrounds, cultures, and native languages are known for their lack of misunderstandings, after all.
I’d encourage you to think about what what your goals are for participating in a public forum, and what kinds of engagement actually help those goals. Revenge on the world for the poor treatment that I received on the Internet, but managed to persist through, is not something that helps me get what I want today, so rather than being proud of thick skin, I try to help others not need it.