No worries, I don't take it as combative. To be clear, I'm not trying to shame anyone who decides that they can't Open Source something they make. There are positives and negatives to these approaches. I'm also not trying to say that there aren't reasonably ethical, proprietary, Source Available licenses out there.
I still use Aseprite, in part because its license terms are very good, even for a Source Available application. In the art world, some of the Creative Commons licenses are far more restrictive than any Open Source code license could ever be. I don't think the Creative Commons licenses are unethical.
That out of the way:
> Can you name a single example of this happening?
Your own example is quite good.
> we have examples like AWS offering best-in-class hosted MongoDB that the mongo guys could never compete with, no matter how hard they try.
There are two ways of looking at this. One of them is that by outcompeting Mongo, AWS is making it harder to maintain the core software. That's a negative. However, it doesn't mean that the alternative wouldn't also have drawbacks.
The other way of looking at it is that in a world where AWS couldn't Freely build hosting services on top of Mongo, we would be losing out on a best-in-class Mongo hosting solution that the Mongo maintainers could never provide at the same level of quality and reliability. The entire Mongo ecosystem would be worse, because (frankly) the Mongo team is not as good at SaaS as Amazon is.
Sure, Amazon could license the software under separate terms. They can license Oracle Database too. But in both cases, the core license isn't Free (Libre). We're now in a situation where companies are competing on contracts, not just on on hosting, and we're now in a situation where the Mongo maintainers can leverage prices or exclusivity to change how the market looks.
Jumping back the the example of Matrix: yes, Matrix is a protocol. But it's also an Open server implementation (Synapse) that anyone can host for commercial purposes. Having Open server implementations means that what hosts are competing on isn't "who can implement the protocol best", or "who can secure the best licensing agreement". It's just "who can provide good hosting and customer service". The organization behind Matrix does provide their own Synapse hosting, but their explicit goal is that eventually their service will be outcompeted and die.
I would contend that supporting that kind of ecosystem matters for a heck of a lot more than 0.1% of developers. 99.9% of developers are not hosting their own Mongo instance. They're taking advantage of the market. Unless you're planning on rolling your own cloud for every SaaS product you ever launch, the market matters. Every developer who uses a 3rd-party cloud to host their stuff (ie most of them) benefits from companies like Amazon being able to freely offer hosting.
This might sound like a technicality, but it's the same reason why a lot of other things matter in Open Source. I have never read the Linux kernel source code, but the fact that other people have makes me trust it more. I've never compiled a custom kernel, but the fact that other people have means I can get custom kernels for obscure hardware. I have never started a commercial Postgresql hosting service, but the fact that other people have gives me confidence that I will always be able to find a quality Postgresql service when I need hosting for my own projects.
Of course, that Open market has downsides in that there are fewer people funding the software. But those are the same downsides we see in many other core attributes of Open Source. Being able to freely compile and distribute programs has the obvious downside that in practical terms, it is very hard to sell your stuff, regardless of what Richard Stallman would claim.
My goal is not to demonize people who say, "giving up an Open competitive market is a fair tradeoff for supporting core software." My goal is not to demonize people who say, "if I release my game/tool/library as Free (Libre), I won't be able to support myself." I think those are reasonable concerns.
But at the same time, I would challenge you -- if Amazon's ability to host Mongo doesn't matter to 99.9% of developers, why is Amazon making so much money from this? Why are developers choosing AWS over Mongo's SaaS offerings? I would argue that the answer is that AWS is genuinely providing a tangible value to a huge number of programmers, and that those programmers really do care about being able to access 3rd-party hosts offering that kind of value.