Even paying for software doesn't guarantee they are going to keep the product the way you like it (OSX, Windows).
This attitude seems to think that open source is some magic wand and that it will be around forever just because it is open source. Open sourced code falls by the wayside all of the time, so I don't think this really matters in the scheme of things.
Some did not like Gnome 3, they created MATE. Some found GCC was too conservative, they created EGCS. And MariaDB. And LibreOffice. And ffmpeg… And…
That's how you guarantee software will continue to fit your needs in the future. It's not paying for Microsoft Works that made people able to open *.wks files with Microsoft Office when Microsoft discontinued the former office suite (though they are some converters available). But it's open-source that made people able to open TrueCrypt files with CipherShed when the developers gave up.
Even at my level, I'm able to tweak the tools for my needs. The last version of zsh is not available on AIX? I can build it myself (and send the fixes upstream). Vim doesn't provide the feature I want? I can add it (and who cares if it's not yet merged upstream? it still does what I want it to do, now).
With FOSS software, an enterprising and capable individual always has the ability to modify/update/or other wise change the software to suit their needs.
I find your line of thought to hold little weight.
Sure, and the parent I was replying to said he is happy to pay money for software he uses. I don't see why paying for closed source software (to encourage the dev to keep developing it and also to allow you to comment on what to improve) is a bad thing just because the project is not open-source.
This attitude seems to think that open source is some magic wand and that it will be around forever just because it is open source. Open sourced code falls by the wayside all of the time, so I don't think this really matters in the scheme of things.