There is not just cost to count. There is also value in having a custom solution that is well integrated into your other tools, and value in avoiding vendor lock-in. Large companies build a lot of their own internal tooling for those reasons.

Let's say there was no open source replacement (that's not true, or at least it's not going to be true when Podman and Rancher improve, but for the sake of argument...)

What would prevent Docker from doubling the subscription right now? Tripling it?