The comment you originally responded to said:
> The model of "copyleft if your project is open, pay us if your project isn't open" is one where I have no problems or concerns, and will happily use the open version and recommend that people building something proprietary purchase a paid license. Nor will I typically worry about the motives or future of the project unless I have some other reason to. And the KDE Free Qt Foundation means I never have to worry about Qt going proprietary.'
Software under a proprietary license is something I can't build other Open Source software atop of that I expect other developers to use and collaborate on. I don't want to have a forked ecosystem of proprietary-with-source-available software, I want to actually collaborate on Open Source software.
With Open Source, I'd feel confident that if we had to manage it ourselves, or fork it and add a patch, or get a third-party to develop a patch, or work with a dozen others with the same needs we have and collaborate on it, we can do so. It's reasonable to build an ecosystem or community or company around. You cannot replicate that with any non-open license; by the time you're done granting all the necessary rights, what you'd have is a non-standard-but-Open-Source license, at which point you'll get more traction if you use an existing well-established Open Source license.
I don't really care about encouraging the development of more proprietary software, whether or not it happens to have source available. There are already well-established models for getting people to pay for proprietary software. If someone is looking for a funding model for Open Source, and what they find is "turn it proprietary and generate FUD that it's as good as open", that's a failure. And when people are looking for Open Source and they find proprietary-with-source-available, it undermines decades of careful work and explanations by the FOSS community, and generates substantial confusion.
It's your software, and ultimately your choice how to license it. Various companies have tried the "source-available but still proprietary" model. Just please don't call it open or contribute to the FUD around proprietary-with-source-available licensing.
Speaking from experience, when encountering software under a proprietary-but-source-available license that tries to act like it's open, the response from companies that actually deal in Open Source is not "Ah, OK, let's pay them and use it", it's "yikes, get this away from us, how did this even end up in consideration, let's either use something Open Source or use something actually proprietary, not something by someone who is either confused about the difference or hopes other people will be". (The set of engineers and lawyers who deal with software licensing professionally, at various companies, tend to talk about it with each other.)