This not "Empire Strike Back" of 1980. The Russos and M&M are not writing fluff. They are producing very mature themes, with semi-realistic circumstances, with fictional characters, in the back drop of a comic book movie. With that said, the appearance to not realize, with that statement, the Avengers inability to stop essentially, a galactic god, an Eternal(Deviant), at overwhelming odds, compounds nothing, when they are eventually unable to restore half of "Life, to the Universe"; when they never was going to stop him from his initial Quest, to vacate half of "Life, in the Universe", in the first place. The Avengers was always destine to fail at such a daunting task...that's the human element, go up against a galactic being.
What make no sense whatsoever, as you put it, fans expectation, the heroes somehow, will restore half of life in a Universe, they themselves, cannot access the endless boundaries of, against a being whom travel the stars, physically, wiping half of life, on an endless number of planets; when the Russos and the writers M&M themselves, whom have set precedence in heroes failing to "completely win" in whatever circumstance they are faced with....from CA:WS, CA"CW, & A:IW, said themselves, this film is not what you think it's going to be; and, those that die in this film, will remain dead. What also makes no sense whatsoever, is the notion that the Avenger would steal Gems from Thanos, create their own Gauntlet, and undue the "Snapture"...when in essence, there have been no evidence or hint, whatsoever, of that happening; just a very good fan speculation, that have circulated the net so heavily, it has become fact, in it's own right. In a matter of a few seconds of video, you have deduced the MCU would be left in a "permanent dystopia disaster" if they don't restore the Universe back to its Pre-Snapture self. There is nothing that says that's happening. We no not what is happening on other worlds in the Universe & honestly, what we saw in just the NY harbor, may just be the immediate aftermath of the Snapture; as we saw, uninhabited helicopters & cars crashing, in a world in chaos.
My point being, Dr. Strange was asked how many did they win, out of the 14mil plus....the answer was one. Now what exactly does winning means to Dr. Strange? Strange made it perfectly clear, on several occasions, his mission was to protect the Eye of Agamotto(Time Gem), from getting in nefarious hands...to the point he told Tony directly if a choice between Tony's life & the EoA, he would choose the EoA every time. That's is a huge sacrifice, if you understand, he probably would say the same, if the choice was between saving a billion people, an octillion of beings, or half the Universe and the EoA, he would chose the EoA every time. Yet he saw 14mil plus scenarios, & every scenario, including the one won...Thanos apparently attain the Gem anyway and fulfill his Quest. I suspect winning to Dr. Strange is not undoing the Snapture….he probably saw a number of scenarios where the Snapture was reverse, with grave consequences. I suspect they will learn to refocus not on restoring the Universe; but, the safe guard(as Strange would) or the destruction(as Tony would) of the Infinity Gems. Tony will be the one to realize this against Cap's undying stubbornness(some move on, not us)….Cap would jump on the grenade as we saw in the CA:FA; Tony would cut the wire as he so eloquently stated in Avengers. I suspect some lives will be saved; but, they will have to make the choice to refocus their approach.
So, I'm not going to go on & on, I'm just saying what the Russos have been stating..."this will not be what everybody think" & what is it that everybody think, that make them say that....you got to ask yourself that. There is a number of other things I could bring up to support my suspicion; I'm just trying to address some of your thoughts that counters my own.
They are not burning down anything; yet alone, "all"....it's just a more mature approach, of the dire consequence of the story.