I really appreciate this. This is getting to a point where I see the possibility of understanding the ontological beliefs which probably led us to our worldviews, and that's something I find very positive.
Paraphrasing you below. Please correct where needed:
It doesn't matter if there is objective reality. Humans can't always know objective truths. Because different interpretations exist. Sometimes humans can know the truth, but not for the complicated stuff. The more people involved, the more objective reality changes. Because different people interpret it differently. Post modernism and quantum mechanics revealed the above to be true. I hope that's not a too large miscommunication, I trust it's not strawmaning you.
I didn't expect to start an ontology argument over this topic, but it's a good conversation to have, because as you have noticed I believe ontological mistakes are at root of the societal decadence that I pointed earlier and the OP described. Do you think this is arrogant? I guess so, BUT any ontological belief requires some arrogance, because these are the kind of truths where objective proof is often impossible.
Onwards, your whole first paragraph, if qualified as per my paraphrasing I agree 95%. I disagree with your implication that objective reality does not matter. Because if it is possible to sometimes be right (not morally, but as regards being literally correct - aligned with reality) then it's worthwhile to try for it. I'll get back to this.
Your second paragraph you half agree with me - that it is possible, not that it is worthwhile, to pursue the truth. Your qualifier about complexity is where we start to disagree more strongly:
First, complexity reinforces objective reality, in terms of averages and observable, repeatable, consistent natural laws. Sure, you might not know what one electron does exactly, but get a million or so together and suddenly you know (and can predict) a lot very well. One could argue the same also applies to humans - a mob being often more predictable than an individual.
Second, the observer does not modify reality through the act of observation. This is a tricky point, where the semantics need be precise. It is not observing, with its implied meaning of human (or conscious) perception, that causes quantum collapse. It is a form of material interaction. The sun "observes" other stars, an atom "observes" another atom, a wave "observes" another wave. There is nothing in the equations about human conscience
deciding which outcome becomes from the probabilities that are feasible. I do agree however, that correctly interpreting reality is fundamentally more difficult when human decisions are involved. I'd actually enjoy if you want to talk about free will and determinism in the brain, anyway reality without humans provides a good enough first ontological model to discuss, and simpler.
So at bottom line, no, lack of agreement does not prove multiple realities are correct. It only proves we are messy and that it's hard to interpret THE reality and be correct. IT does not depend on how you measure, only your interpretaion of IT does.
And here we get to the crux of the matter. On quantum mechanics I have commented. My interpretation may be incorrect, but I believe it. Postmodernism... Postmodernism (as normally understood) is arrogance to the power of 3. It is arrogance that humans determine objective reality. It is arrogance that humans modify objective reality. It is arrogance that objective reality is.
That objective reality is, I have no issue with. I am likewise arrogant. The alternative is to believe this is an illusion, but if so, what or who is deluded? Cogito ergo sum and all that...
That humans modify reality, likewise no issue. I won't say much - but I believe our vertigo at the technological power we have is one of the root causes for the following. Like a child being cruel, choosing to believe she isn't. Basic denial...
That humans determine reality... Well, we might have killed god, but I wouldn't go so far as agreeing we replaced it. :) The logical issue with this position is obvious. Given how you correctly pointed out that we can't agree anything as groups, who exactly is determining what? I guess when I die the universe will cease to exist, and before I was born you all popped into existence as decorative elements. Or there are superimposed subjective realities which collapse into... something objective?
This position is logically inconsistent unless one assumes some form of extreme solipsism. And here we get back to the ought argument. For me postmodernism is incompatible with a lot of moral concepts. I mentioned a few when we started this dialogue. The critical one: how can you tolerate or respect another that is part of your subjective reality yet not objectively real and independent of you? You can't.
If you really believe reality is objectively subjective ;) then you believe everything is SUBJECT to you. Consider the etymology of subjective here, I find it very revealing of the power game being played, even if unconsciously, by most postmodernists. It's all subjective effectively becomes it's all mine.
So there you go. Trust that provides enough detail to justify my position. Believing in a unique accessible objective reality is a requisite for correct morals. Otherwise there is nothing to measure, and nothing to measure with. Without measuring there is no scale and no comparison, there is only ego. It's ALL mine.
Of course, agreeing to this is not the end of the story. If you do, we can get back OT around the politics of being correct and how to find what is true - when it is even possible. I expect we would agree more on that, and having the recourse to objective reality will permit us to be... objective. ;)
If you don't agree, then I'm sure I'll enjoy your perspectives. I can't say I have a lot of respect for the usual conception of postmodernism, and if you can change my mind to any degree I will gain from it.