1. Well, this is the root of the conversation around free software, especially in the modern age. I think many people would argue that Windows/Apple/Android users are indeed computing, but they're doing so on the behalf of Microsoft/Apple/Google. When those companies back up your files to their first-party cloud systems, that's not
your computation anymore. They have your data and can ostensibly do whatever they want with it. There may be quibbling to be had on this topic, but I think the original point still stands in context.
2. I wouldn't call it a prison, but it would be pretty disappointing to move into a listed building where the thermostat was locked behind an acrylic case. Simply put, it sucks when you pay for something and can't run arbitrary software on the hardware you own. I don't really think most people would disagree here: having options is better than having none.
3. Is it really a fantasy? I doubt the average PC owner even knows they have options other than Windows, much less free ones. Maybe this is the HN US-centric mindset taking over again, but nobody in America really knows or cares about this stuff, for better and worse.
4. Pretty much all of your refutations so far haven't actually debated their core point, so I don't think it's fair to start throwing stones. Given the average vitriol of HN, I would say that many different people are "often demeaned, or neglected, even here on HN", be it Free Software advocates or proud Macbook owners. It doesn't make them any more or less credible, but their original point still stands: people on this website love to quibble about how atrocious OSS is when compared to $GLORIOUS_SAAS, despite the fact that the iPhone/Windows/Android device they're writing on is more OSS than it is proprietary.
Whether it's Apple, Microsoft or Google, none of them deserve to inherit the future of computing. It's not a radical opinion, and I'd agree with the parent comment that it's pretty often overlooked on a forum about startup culture and business.