1. Entity donates resources to maintain a resource for free, while pulling in revenue from an unrelated source => they're beholden to that unrelated source and it's unsustainable, we shouldn't take them seriously.
2. Entity scales back to maintenance of that resource => they're abandoning what made them great.
3. Entity re-monetizes the resource more directly => what are they monetizing, they don't do anything to maintain it.
What people want is an instantaneous jump to:
4. Entity already has resource monetized and is already significantly maintaining that resource.
But a company in position #2 can't just jump directly to #4. It's fair to ask about the direction that a company is going and whether or not they'll follow up, but sometimes I feel like critics want teleportation, not movement.
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Mozilla is pretty clearly still investing into MDN (both in ways that I really like such as the learning areas, and in a few ways that I'm less thrilled about, like a few recent UX decisions). But if MDN plus allows them to continue that investment, it's worthwhile -- ideally, if they make enough money off of it, we might see them increase that investment. If there's evidence that they're not going to, then fine, I guess, but I don't really see that evidence.
What MDN Plus offers is basically what people have been asking for with Firefox except for MDN. It's direct funding for the product itself.
I'll also point out that providing a platform for permissive-licensed content is itself important work and should be supported. It is good that this content is permissively licensed, and alongside MDN plus, we can actually look at permissively licensed donated content as a way of "funding" a public resource. If the content wasn't permissively licensed, my feelings about that would be very different, but this isn't a scenario where people are donating resources to Mozilla that only Mozilla can use and that are then kept captive -- people are donating content that anyone can use and that anyone can modify and re-host, it's remaining in the control of the community.
That's not to say that we shouldn't try to get to #4 again, but an MDN without a ton of professional editors is still worth funding. Particularly given the contribution model, where if you really want to pay for editors you can just go hire editors yourself and pay them to contribute to MDN.
This reminds me a bit about the conversations about Wikipedia. I have tons of criticism about Wikipedia and tons of criticism about how it fundraises, but one of the criticisms I don't have is that it has too much money. Wikipedia is one of the most important resources on the entire Internet and it's good for a project like that to be over-funded. Similarly, I think MDN is one of the most important educational resources for Javascript on the entire Internet, and I don't really see the problem with giving it more money, even if all that was happening with that money was that it was being dumped into server resources or making the owners feel more comfortable about it.