Barca brought in both Bravo and Ter Stegen because they wanted to have two world-class keepers at the same time. Barca is a very rich club, plays the heaviest schedule of any team in the world, having two provides injury/suspension coverage.
Bravo has been fantastic: he has won every competition he has played in for two years, 2 Ligas and 2 Copa Americas. This is the best record of any keeper in the world over this period. Ter Stegen has not looked quite as good, but it is harder to be consistent not playing enough, he is still developing, and a CL and two Copas is not too shabby.
So, as we have seen before at other positions, this quality of depth is great to have, but players of this level just won't put up with it for too long. These two are forcing Barca to pick one, Barca seems to be going with the younger. Not surprising. Inevitable, really. I hate to see Bravo leave, but Ter Stegen is (I hope) the one to go with. Now, Diego Alves to be #2, please, Masip will never be good enough.
Bravo will be a big improvement for City, Hart fanboys just have not been paying attention for the last couple of years.