Good point. It's not that clear and easy in all cases though. Besides, Stallman once disparaged SaaS business models as unfree and essentially a problem to be dealt with. While that in way makes sense from his point of view, it's problematic for developers who try to make a living with providing a useful service.

I very much agree for all system software and especially cryptographic and security-related software.

What about a developer or a company though that's put several years worth of work into developing a particularly useful novel solution to a specific business problem? If they use a proprietary license they'll at first force others into depending on them once they start using the software but is that really wrong in every case? If on the other hand they immediately release their code under an open source license they might effectively commoditize their novel solution. Sure, they might be able to recoup some of their up-front investment but in most cases this won't be a sustainable business model.

Releasing the source code only to paying customers might seem like an obvious solution but depending on the nature of the software and the industry the customers might decide to just pay once and afterwards sideline the developer. Continued innovation can serve as an incentive to pay for updates but again this doesn't apply to all fields and industries. Licenses such as AGPL might be a remedy, too but from experience I can see that software licensed under these or similar terms is mostly avoided in business contexts. I'm not sure if there's any company that's built a viable long-term business on a dual-licensing model.

Not every closed source (or only partially open source) software product is created with malicious "Let's blackmail the customer into using our software forever." intent. Sometimes, it just makes good sense. However, in those cases I think companies should at least make provisions for releasing the code if they go out of business.