Looks like you have never tried to monetize FOSS.
It's close to impossible. Look at heartbleed or the xz disaster. Two fundamental pieces of software used on billions of devices, neither of them made enough money for a single full-time dev.
And you are suggesting that one can make enough money from a FOSS REST client?
Look at breadsmasher's original comment. There's not a word in there about "betraying FOSS ideals", nor does it contain a complaint about "closing the source".
No, the whole comment is "Fuck Thunder REST because they want me to pay for features". Breadsmasher is a freeloader. He would never donate anything. He uses FOSS, not because he wants freedom as in free speech, nor does he use it because he wants to review or modify the source. He uses FOSS for the "free as in free beer" aspect.
And when someone says "Now please contribute to a project you've been using for free", he gets entitled and throws a fit like a two-year-old.
This here. FOSS as a dedicated concept exists since around 40 years and we still haven't figured out how to fund it.
The only meaningful way to fund FOSS is the Linux way: Coprorations use the FOSS in a way where they can't sell the software directly, instead, the software is used as infrastructure to make money another way.
Microsoft, Amazon, Oracle and so on, they make money by selling cloud compute running Linux. To do so, Linux needs to be really good, so they contribute developer time too the Linux kernel and to certain tools.
The same strategy works for programming frameworks, like Angular, which is being developed by Google, because Google uses it in tons of their products.
But this doesn't work for end customer software and it's basically impossible to get a "grassroots" developed piece of FOSS into the corporate mill.