Lots of goodies, Cap!

The strike for is that no one currently alive will ever be able to tell them that they were wrong
Yes. But it does depend on what they're trying to achieve whether that matters or not. I don't think many SF writers are genuinely in the prophecy business, so probably they wouldn't give a damn either way, as long as people buy their books. All else is a bonus (that they won't know about). Oh, and probably the writers would rather people enjoy reading their books, so they can make a living writing.

The strike against is that if they wish to be taken seriously by intelligent readers they can't posit basic changes in human nature or the way that society works
Hmmm, generally agree with you here, Cap (as I generally agree with you that basic human nature won't change) , but it is just a matter of degree, certainly not 100%. Even Asimov, many years ago, wrote about an alien society based on three sexes. What relation does that have to anything we know? Did it adversely affect sales of that book or subsequent Asimov books? Not in the least. People love to be entertained and many love to have their imaginations stretched and thoughts provoked. In that context anything is fair game. Creating coherence and plausibility is difficult, but certainly not impossible. Again, it's not really about prophecy.

I feel SF books about parallel worlds are quite relevant here, as they can show how very different our world would be following an often small historical twist. And where do you draw the line between basic human nature, and changeable human nature? Same applies to the way society works. I agree there is something fundamental that appears to be in us and our society regardless, but it's bloody difficult to pin down what that "something" is.

They merely take current social structures and mores and impose a "superior" or "advanced" technology on them, calling it "the future". They don't ring true for me

Certainly true of Star Wars, although that is (again) more of a myth/allegory - "a long time ago..."
Dunno how much you could really call it projection of the present though.
Then-current social structures and mores did and didn't apply to Star Trek - the tolerance and (sometimes) appreciation of significantly different cultures was a very important aspect.
All I know about Starship Troopers is that I enjoyed the book as a 12-year-old, and enjoyed the film fairly recently. Plausibility? Not bloody likely! Present projection? Too right! Messy jingoistic childish fun, totally unworthy of a mature adult? Yep.

One series that fascinated me because it tinkers with institutions is Greg Bear's Eon trilogy
I'll check it out some time.

I liked Bob Shaw's Vertigo for its depiction of a society changed by anti-gravity harnesses. Nothing essential changed there, though, really. How about his "slow glass" stories? Maybe the same. I keep coming back to Moorcock's Dancers at the End of Time.

The one thing I can say for sure is that whatever any writer predicts, I predict it won't happen that way if they are projecting more than 20 years into the future

- Or a lot less! These are very interesting times to live in, to quote the Chinese curse. Although it is definitely something to watch the future happening.
Something wonderful.
Something absobloodylutely terrifying.