Why Quicksilver should be kept in MCU?
Well first, i think that wanda/scarlet witch/quicksilver's sister has a unstable emotion control and she need someone to look after her. Maybe vision could be that someone, but her brother is all that she has when their parents died, right? I think that quicksilver knows wanda pretty well.
Second, MCU need heroes with a family bond. To give an example about how heroes should treat each other. Quicksilver is a young hero, he can add more young adult comedy with spiderman in the MCU.
Plus if quicksilver lives, avengers would have gained another cool superhuman, especially in the upcoming [The Avengers: Infinity War](tag:738027), the war would be more epic!
That's my opinion, sorry for the bad english. Peace out!