To add to Yoricks comment:

Many of our customers have large MySQL clusters running. It's very convenient to be able to just plug GitLab into that.

We have no intention in dropping it in the near future. For instance, our Pivotal Cloud Foundry (PCF) tile makes use of the MySQL cluster in PCF [0].

For those reasons, we'll continue to support MySQL and make sure it's performant.

[0]: http://docs.pivotal.io/gitlab/