The world is going to open-source distributed systems built on commodity hardware, which Red Hat has done a great job of building a business model around. For old established companies, though, the migration is a lot of work, and there are _definitely_ still plenty of large companies who have purchased/leased mainframes for long periods of time, and would like to modernize, but also can't afford to throw everything away and rebuild from scratch. There's a lot of work being done already between the two companies to run Go, Docker, Kubernetes, etc, on mainframe, and for companies with mainframe resources, being able to get a little more utility out of those sunk costs is very attractive, and something Red Hat's expertise has (and would continue to, presumably) help accomplish.
That being said, I'm pretty surprised at the news, and I'll be watching closely to see how things go.