I doubt their infrastructure would scale well, even with more income. They still haven't adopt WebSocket/Rails 5, and this would attract more CI runners to DDoS their database.
Just wanted to let you know that we introduced long pooling with GitLab Runner version 1.9.0 (~ three months ago).
Also you might want to take a gander at https://about.gitlab.com/2017/04/10/upcoming-runner-changes-... We're throttling older runners.