The point is simple. Putting the most common container management tool behind a payway (and potentially the most common local container runtime, in the future) will drive down the use of containers across the board.
Sure, there are tools, but how many of them work consistently cross platform? How many are as easy replacements as a drop-in replacement to docker?
Imagine a team of developers that are merely users of containers where adoption was driven by another more knowledgeable person. If the company won't pay for Docker, do you think they will invest in time to rebuild a development environment from docker to something else? Developers getting familiar with another toolset? Sure, it's all OCI, but the tools are different.