Exactly.
Dam Busters was not an accurate portrayal of the exploits of Barnes Wallace and his team any more than U571 was an accurate portrayal of the capture of the Enigma Cypher.
It's not the movies that have changed, it's our knowledge of the facts.
However, because special effects have got more spectacular, there is a tendency, not so much to rely on them more to tell the story, but to bring them more to the foreground. Clash of the Titans was considered, in it's time, to be one of the greatest SFX movies of all time, yet it just about equals Pirates of the Caribbean for SFX contents these days, and PotC is, in my opinion, a far better film.
What has changed is not so much the content of the movies, as the promotion of them. Films cost a lot more to make these days, so there is a lot more emphasis on making a profit.