An ecosystem has evolved around Borg. Custom hardware, kernel, schedulers, telemetry, atomic clocks, networking, security, management... have all evolved around Borg proper to meet the "enterprise" needs of one of the largest enterprises in the world. It has many niche (i.e. not generally useful) cababilites built to service hardware melting XXX megawatt applications like web indexing, gmail, colossus. Even if Kubernetes targeted this customer, Borg has a significant head start. At this point the Borg ecosystem is almost old enough to by cigarettes in Mountain View.

Assuming that Borg it's proprietary (IE, zero documentation, no community, no real user cases outside of Google), yes. As all Google projects, let's wait its complexity doesn't collapse under its own weight, leaving no options but to move to somewhere simpler, which isn't on the horizon.

That sounds like a technical question that was given a political answer. ("Kubernetes is so great we don't want to keep it all to ourselves here at Google, let's give it to an open source foundation!")