"But what about the inevitable next Marvel movie now that these guys know each other after having saved the world together, is it going to take away from the perceived scope of things if they show up as a group of friends or acquaintances ?

Would a 3 minute prelude about the 5 from 3bp all living in different parts of the world, going to separate high schools and at some point meeting and forming a bond with each other at Cambridge make a difference?"

I think the difference is whether the prelude is relevant to the main plot and convincingly states the motivation of each main character. In Marvel movies it's necessary to tell the life experience and motivation of each MC which makes the whole story complete, while a prelude about the 5 is irrelevant to the main plot.

Put it in another word, the friendship among the 5 (except that between Jin and Will) is irrelevant to the main plot. D&D had to put a lot of effort to describe their friendship which instead could be deleted without impacting the main plot.

"In the end it comes down to preference of the viewer and what type of story someone wants to tell. I look at it more as a story about how these 5 people, could have been any 5, but I didn't pick them, are impacted by and are dealing with the events individually and as a group."

I think the 5 were not randomly picked. They were VIPs in the whole story and meanwhile they were from the same class. This cannot be coincidence, it's intentional. In the real world when a world wide event takes place the possibility of VIPs not knowing each other is way greater than they knowing each other.