We don't know for sure that Chris Evans is leaving...
We know that his contract will be fulfilled very soon. That he's expressed interest in other projects. And, if you're at all familiar with the comics, you know that it would be very natural for the actor to part ways with the character at the end of Civil War.
So... who would you like to see next?
Marvel has some great options on the table. Bucky and Sam Wilson have both carried the shield in the comics.
Or they could cast someone new.
Same character, different actor. It would be a little weird, but give it a couple of years. They could reboot the character. Maybe focus more on his exploits as a soldier during WWII.