This is really just a "wrong" way of looking at this film, or really any film, as far as judging the quality of the film goes.
I have seen people make this argument, and it's silly. It makes a 'damned if you do, damned if you don't' situation. Either the good guys win, like they always do, thus we have the "predictable hero movie", or the bad guy wins, but, "obviously it's a fake out and they'll just deux ex machina it in the next movie" There is no winning if you're thinking is that limited.
The mistake people are making is thinking about all the outside factors that have nothing to do with the actual movie. The movie did not end the way it did as some sort of stunt, or "fake out". That was the story. You weren't supposed to be thinking about actor contracts, and what sequels are on the horizon for individual characters. You were supposed to be in the moment of the story. The characters don't know that they have sequels. You're supposed to be feeling their story. You guys are ruining the movies for yourselves by over thinking them
And the Russos said that this movie could be a standalone movie, and they're right.
If somehow the already filmed Avengers 4 movie disappeared, along with all the actors, this movie could stand alone as the end of the MCU as we knew it.