This is
incredibly shortsighted, but the rebuttal is simple:
> This being the case, who cares if GitHub is dominant? Or SourceForge, or whatever the next popular thing might be.
For one, I do, and for two - why was sourceforge changing their business model to shady predation a problem? It was not because they were a website on the Internet - there are millions of URLs of clickjackers or other malware you could visit and harm your computer with, but you rarely see them.
The problem was that a ton of open source software had mindshare on sourceforge, that remains a problem to this day, and it took years to move most projects away from that hostile environment. Plenty of very useful free software remains hosted on sourceforge, and there are plenty of reasons - developer inertia, community loss from switching, legacy systems in place that aren't portable, lack of interest in learning new tools, and many more - but the reasons matter less than the result - that we have thousands of projects staying on a website known to infect people with malware.
Most of that software remains as portable as anything on gitub - often even moreso, because sourceforge offered many fewer developement ecosystem features than github now does - but has no switched for whatever reason. We can go after the individual projects and heckle them until we can get them off sourceforge, but that is a ton of effort and mental energy we could have better used making good software.
Which is fundamentally why the decision between github and any open source alternative is so important. This is not a question of benevolence, or even time - Github, Inc is a private company hosting a proprietary website that has 11 million accounts and 29 million source repositories today. Any action they take can destroy either trust in the platform (why exactly do you trust a proprietary web service, again?) or its usability for whatever purpose you depend on it for, and since it is proprietary there is no recourse. You just have to repeat the sourceforge hell and somehow move off of it as a hosting platform - except you might have drunk the koolaid, and now have your issues, releases, build service, wiki, and website all bound to the github platform. If moving just the source control, release hosting, and a forum from Sourceforge was bad, trying to get away from Github would be much, much worse.
But all these migrating projects should know better. They were already betrayed once by proprietary software they depended on, but are taking familiarity and mindshare over the security to never have that happen again. At least with gitlab, when Gitlab Inc jumps the shark, you - or anyone else could spin up the lifeboat to easily and seamlessly save your community with. And that collateral alone means Gitlab Inc. is much less likely to betray you for profit.
It is not a question of if. Unless Github open sources itself, and that is impossibly unlikely considering how huge their code base must be now and how many football fields of lawyers they would need to prune their internal code, a proprietary software project must eventually act against your interests because you are not in control of it, no matter the intention of the creator.
If you are going to have to bite the transition bullet, you might as well only have to do it once. Considering the parity between github and gitlab, I have never met anyone who would literally refuse to contribute to a project because its not on github, you just miss casual eyes that are more common there because the platform has captured more userbase.
But that userbase control is so dangerous, and we should all care enough to try to correct for it when we can, if it doesn't negatively impact us much. And honestly, a project like Python would have been perfect for it - they won't see a dearth of developer interest just because they are using the second most popular source control web service out there.