The first 70% of the movie is 10/10

Different takes on it, I guess. One key phrase in the film is "small steps" -- plenty of people believed her, and humanity must come to grips with the knowledge of other races across the Universe. That doesn't happen overnight. Humanity must change & grow to be ready to join the community of those other races in peace. "Small steps."

Perhaps you're being a little too literal here? One of the themes of the film is faith: what one believes in & why. Clearly a great many people believe Ellie, as shown at the end of the film. And how many things do we believe & accept about the world without ever having personally seen or experienced them ourselves?

How many scientists believe that there's other life in the Universe, without one scintilla of actual evidence?

Ellie is believed by many because of the obvious emotional impact of her experience, her utter sincerity in attempting to describe it. They may not have shared her experience, but they can see for themselves that she did have a powerful, life-changing experience. That alone is enough to open their minds to the possibility of its truth. She isn't demanding that others believe, she's simply saying, "This is what I experienced." Acceptance or rejection is then up to each individual. And a great number of people accept. Just as some don't, because they need the irrefutable evidence in their hands first. Both are valid responses.

OK, I see the people in the movie as behaving as real people would behave. The mistake, I suppose, was to broadcast the Vega message to all and sundry, resulting in the "carnival" atmosphere they had both at the VLA and at Cape Canaveral (?) for the test run of the Machine. The resulting tragedy of the crazed fundamentalist bomber blowing up the first Machine was the result, so they had to change tack and massively improve security.

And if the plans for an alien machine were suddenly presented to us, they wouldn't waste time in using it. What I said about acclimatisation was what Star Trek does, not what this movie would do. HOWEVER, aliens themselves landed on Earth, in front of dozens of witnesses, so the aliens forced the issue and left us no time to acclimatise ourselves. True, humans flew a unique FTL spacecraft that got their attention, but the humans did quite well to get used to the alien visitors, considering they had some warning by future humans.

What I'm saying is, in both cases, we let aliens know of our existence (Contact's Hitler broadcast, Star Trek's Phoenix) and they replied. We've been waiting for aliens for decades, and I think we have some idea what to expect.