> Protecting the user's right to compete with the developer is not sustainable.
I agree with you, actually - but Richard Stallman and the Free Software movement more generally really don't. They exactly and explicitly believe this right exists and should ideally be a legal right, and the AGPL quite explicitly maintains this right.
Ultimately the Free Software movement is predicated on the concept that ideas can't be owned. They generally oppose both copyright and patents, and not just for software. Their licenses are meant as a stop gap solution. Ideally to them, or at least to some of the more die-hard members, laws would be changed such that what the GPL grants would not be a license predicated on copyright, but instead a legal requirement for all software, while copyright would be entirely abolished.
In addition to their general opposition to copyright and patents, Free Software people also view software as having a special role in terms of privacy and control - that, even more so than books and other copyrightable works, you have a right to know what the software in your house and business is doing, and to modify and fix it if it's doing something you don't like. This is related to privacy rights on one hand, and also anti-monopoly, right to repair concepts on the other hand.
This is all very different from the Open Source movement, even though they basically use the same kinds of licenses. The OpenSource movement is more of an industry group that believes competing on building much foundational software is a waste of resources. Instead, they believe the best way to build this foundational software is in collaboration with other commercial or non-commercial entities, building it in the open such that all may benefit from contributions and add their own contributions. However, the Open Source movement is completely fine with, and even expects, then making a proprietary product on top of this open source base.
To them copyleft licenses are a tool to make sure others don't keep their improvements for themselves, but have the downside of making it harder to build your proprietary stuff on top. Conversely, software that takes your contributions but then doesn't allow you to use it in commercial offerings is completely unacceptable, since the whole goal of the movement is for different companies to build a common infra on which they can then build their own commercial products.
Ultimately, both the Free Software and the Open Source movements will agree that a core part of open source is that anyone should be able to compete on delivering the original software, even if for entirely different ideological reasons.