> I may never have to use Terraform, Kubernetes, Docker, CoreOS, and all that other stuff.
True, you may never have to as long as you're okay putting 100% of your faith in the benevolence of a single company. Remember, a good portion of AWS services are completely proprietary and serve as a pretty fancy set of golden, jewel-encrusted handcuffs.
Fully open source projects like those you mentioned, on the other hand, are positioning developers who know them to treat the cloud providers of today as just another source of CPU cycles, storage, and networking. For the services I run on Kubernetes in AWS, I can lift and shift those in a day if/when the day comes that Amazon is a little less benevolent with their pricing and service levels. If we were asked to do the same with our services built expecting AWS functionality, it would takes months to years. Guaranteed.
Yes, there's a trade-off that I don't get 100% managed services and I have to know a little about the workings of the platform, but I would make that same commitment to AWS the second it didn't do everything for me. In my experience, that moment usually occurs somewhere around the 10-minute mark.
Amazon is great today, but Bezos won't live forever and who knows what happens when his successor takes over. It could be great, or it could be a mass exodus of people dumping years of investment. Personally, I'd rather hedge my bets and use their low-cost VMs as my source of compute today, but leave myself plenty of dead-simple exit options and sleep a little more easily at night. That's what open source software is all about.