And the first part I very much disagree with, first, lets both agree that if something is fiction, anything is possible when you are the author. If you write something set during WWII you could have Aliens come save the day or any number of other scenarios.
Most writing, doesn't matter if you have magic or not, adheres to a set of rules, and if those rules are broken within that story, it takes people out of it.
The start of a movie, franchise or series will set the tone for what we can expect.
In a serious racing movie like Senna, we expect a different level of realism than we do in Fast and the Furious 7.
Fantasy generally comes in 2 or 3 types, depending on how you look at it, you have child/teen fantasy, this focuses more on fantastical elements, it's usually not internally consistent, but can be a lot of fun to watch.
Then you have mature fantasy, by this I don't mean mature people should like it more, but this type of fantasy generally sets out with a premise like "we are on a different planet, there are Elves Orcs and Trolls here" after accepting the mutual premise we expect it to be a place where things should happen as they would if the place was real.
There are often discussions within this last type of fantasy if certain things would really happen or not.
The original trilogy would be cheapened if superman showed up to help Frodo destroy the ring or if Gandalf could cast a spell powerful enough to destroy Mordor.