Why are you saying it does not matter, but it does lol. It does make sense, with plane crashes they have found blood, and used some of it to identify the crew. Science has done this, so its not some I'm riding on unicorns with Harry Potter impossibility. How high are the chances that blood will survive? It depends, is there any fuel left on that plane, maybe not, back end and wings torn off, there may be nothing left. So it may just thud to the ground making a gooey terrible mess. Or hit the water and do the same thing. The chances may not be really high of this, but again it does make sense, and is not some impossibility that some are making it out to be. People have done it, it's not some fairy tale. So again maybe a lower possibility, but a possibility none the less.
No it is just odd that something that can happen some act is if it is an impossibility beyond anything. It is not always the case.
As craig said, yea there are elements of , I would not use the word fantasy, but just improbability. Perfect example is Batman. In his 10 year crusade, there is no way he would make it a few weeks, he would be dead most likely. With men shooting at him all the time running from roof top to roof top. Doubtful anything like that would happen at all. The tumbler even if they could get it to jump, it would have gone through most of those weaker buildings lol. But the transfusion is not an impossibility, after the blood they would try bone DNA and learn the truth probably.
There are many elements of realistic scenarios in his film, and there are many that are not. This one is not an impossibility.
EDIT: There are many cases of finding blood even after fiery plane crashes. Here is an example from 1986:
http://articles.chicagotribune.com/1986-03-14/news/8601190483_1_plane-fishing-trip-blood-samples
Rare possibly, but it does make sense and is not an impossibility.