I don't understand the point you're trying to make. Are you claiming that if Mojo was open source earlier, everyone would still be using the old versions for a long time? I don't buy the claim that a pre-1.0 language only a few years old without widespread production use is comparable to one of the most widely used languages on the planet that had been around for close to two decades at the time of what you're describing.
Even if I did think that was a comparable precedent, that still doesn't address the more direct point I made about marketing the language at all if this was a concern. What does being closed source do to prevent people from running old versions of a compiler to avoid updating their code?
It prevents endless discussions on github, hurt egos when my precious PR is rejected by committers, and commentary on not being true open source because they are not welcoming source contribution, feature request, when they are trying stabilize compiler and so on.
Now none of these are guaranteed to happen when it is open source. But at least it is guaranteed to not happen when its not open source.
I don't understand that argument at all, unless you think that the only value of open source is in the potential for people to say nice things about your project. If you think there's inherent value in something being open source, you don't even need to have a Github repo (a tarball on your website with the source code is open source), and nothing anyone says would take away from that inherent value.
I'm firmly of the opinion that there's inherent value in being open source. I would honestly expect most developers to recognize that nowadays, but maybe I'm wrong. My concern is that someone who doesn't think this is the case is probably more likely to also not want to come right out and say it, because that would potentially prevent them from getting the value they're truly looking for from the stuff they do open source. The justifications that are being given here is that they seem to be heavily predicated on the value of open source being primarily from marketing rather than the very nature of code being open having value on its own, and I would be both surprised and disappointed if that's the opinion of the people in charge of Mojo. I'm sincerely hoping that there's something else going on here though, so I'm trying to give the benefit of the doubt and see if I'm missing something.
Okay, but by that logic, if I can point to one instance of something suboptimal happening due to software being closed source, then it's reasonable to be wary of that too. I don't see how this is a plausible justification for keeping something closed source in the absence of taking into account the likelihood of something similar happening here, and for the reasons I laid out previously, I think it's borderline absurd to claim that was a real concern in the timeline that Mojo was closed.
People choose what to worry about. You can say it's unfair because they could've worried about a different set of things, but at the end of the day it's their choice.
I'm not saying you can't worry about this; I'm saying it does not seem to be a particular compelling reason for Mojo being closed source for so long, and I'm dubious that the rationale was due to anything like this.
Even if I did think that was a comparable precedent, that still doesn't address the more direct point I made about marketing the language at all if this was a concern. What does being closed source do to prevent people from running old versions of a compiler to avoid updating their code?