See... I think the unquantifiable aspect of as has been mentioned, fate is what's taken away from the whole "everyone has to be justified in a totally logical manner" approach.
As stated, this makes them a cohesive and indeed professional team before getting powers. One again... Replacement crew member or not... How smart and together is Johnny supposed to be at, what, over 24 if he is in some organization and qualified to go on a "mission"... But he's still reflective of the Johnny we want? How much of a scientific whiz kid is this in his 20's guy supposed to be? He'd have to be a GENIUS. Even to be a back up. Maybe ESPECIALLY if he's a back up. As you conceptualize Johhny Storm, the Human Torch of the Fantastic Four... Could you ever seriously see him as say a character in APOLLO 13? Either in the capsule hurtling towards the moon or at mission control? And if he was and acted ala Evans' Johnny... I would have serious questions about the screening process at NASA.
Fate, not mission requirements brought these four highly individualistic personalities together in a, dare I say, fantastic way. It has always been a contrivance that even the comics have had shots at attempting to explain away with varying degrees of success. (If we want to apply reality to the whole thing, how does a genius so devastatingly underestimate the requirements of radiation shielding in terms of space travel? )
Sue and Johnny in my opinon can be worked into things with far less need to tinker with their backgrounds than others think is my stance. How they got on board the ship is of less importance than in showing the audience how it changed them and what they do with their powers after. And there are issues, like Johnny's classic personality or the dynamic between Reed and Sue which make less sense if you make either people who by the necessity of being in some kind of official government program of some kind highly educated professionals.
If Johnny is a legit kid he has more of an excuse and reason to be an impulsive, even unthinkingly reactionary and often ignorant hot head. Because that's what teenagers often are, but which is the polar opposite of anyone that's being looked at for any official space exploration project.
Sue is in fact more amazing post the cosmic ray bombardment because she wasn't already a highly educated medical doctor that also happens to qualify for space flight. She can be amazing because she's out of her depth but manages to still be a strong voice in the FF and in fact it's most powerful member.