I agree in principle, but trademarks are very useful for preventing others from co-copting and corrupting something. Agile 2 is trademarked to protect the idea set from being co-copted and republished as something different - perhaps something proprietary. Agile 2 is published creative commons, open and free for all - to extend or whatever purpose, as long as "Agile 2" is not claimed to be something else.

But I think your point about trademarks is about when they are used to make something proprietary, and monetize it for, say certifications.

Just as what happened with Scrum. But look at what happened there: they did not trademark Scrum or any of its terms, so the Scrum Alliance sued Ken Schwaber for using the term "Scrum Master". Lesson learned.