why do so many discussions turn contentious?

No no no!! that's the new word for being a fuser  "Pathetic"😵

Apropos the "technically informed," I think the key is exposure. You have to have seat time; all the engineering including audio or circuit design knowledge in the world will not translate fully into what you hear in a given system. If it is good, you want to know why. If it isn’t, same question. I think the value of having a lot of input from users who have spent time with equipment, given their stated controls, may be more valuable than measurements or the theoretical opinion of someone knowledgeable in the art but at a remove.
That is largely the value of this forum as far as I am concerned. (There is also troubleshooting, not really limiting what value there may be to others).
This place isn’t necessarily where one goes for deep on music, although there are a couple of long running threads and the occasional post; instead, it’s mostly gear centric, given the nature of the business to which it is attached. (kinda obvious).

Apart from known synergies with commonly used (don’t read: cheap) equipment, there are a lot of questions lurking in the interaction among components, something that seems to be addressed almost by happenstance.
I’m actually glad for it, because from my vantage point, I have yet to see a "whole picture" (holistic) philosophy in home audio that is satisfying to all. And if we are spending time here to learn and improve the reproduction quality of an audio system, that time ought to be well-spent.
I guess point 1 is that sometimes, it isn’t solely the province of the engineers, though they are vitally necessary and 2, the real key is to hear stuff in the wild, put in place some controls if you are considering outlaying real money (whatever that is for you) toward a component, and have some experience with the limits of what a good reproduction system is capable of, before signing off on a chosen direction.
I’ve found that choices made a long the way often dictate other choices toward a certain end result. Perhaps a different way of saying "everything matters," but to me, each component has some coloration or character that you are matching with other things to result in the overall sonic outcome from the system. Those choices may pre-determine (depending on your objective) the next step in many cases. (Apart from wholesale changes to entire systems, which are costly, at least at the higher end of the scale). It’s impossible to speak about this in the abstract, but it is the area where I think the forum has the most value. (to me)

why do so many discussions turn contentious?

Here it is seeing some can’t see it.
I believe people are just getting sick of those "shysters" that are making big money with "very dubious" cheap products aimed at buyers that can’t tell the difference (the gullible) that fester here, on tv, in the press, it’s everywhere and it’s growing bigger and bigger. (open your eyes)

Can’t we play nice, share OPINIONS and OBSERVATIONS, realizing that they often are subjective and biased.
"if you dont have anything nice to say, say nothing"? If you wish to disagree, do it in a constructive and mature fashion, no need for "argument ad hominem"...

Your talking about a perfect world, where the shysters wouldn’t arc up when asked for proof or were given proof that their products are just designed to make them the max profit from the least outlay from the gullible they sell to.

This is turning into quite the soap opera with some over the top ham vying for the spotlight.