The original Captain Marvel in the comics is a Kree warrior named Mar-Vell; Carol was known for many years as Ms. Marvel, and then later by Warbird, before she finally decided to adopt the Captain Marvel codename herself.
In Marvel Comics, Monica spent a period in the Avengers as Captain Marvel as well — technically, she went by that codename before Carol Danvers did.
As part of their big announcement of their full Phase Four movie schedule, Marvel unveiled the new title of their Captain Marvel sequel.
In other words, there are a whole bunch of Captains Marvel around, so The Marvels makes a lot of sense as a title.