It’s because the current version has nothing wrong with it. If the Lemmy devs should choose to sabotage the Lemmy software, you’d be surprised how easily that happens when it pisses off all the instances and their owners. Instances will simply refuse to upgrade. And like most things, eventually some fork will win the race to become the dominant fork and the current Lemmy devs would be essentially disowned. Different forks also doesn’t necessarily mean API breaking changes, so different forks would have no issue communicating (at least for a while).
First: They did actually end up removing this and making it configurable, check the bottom of the page. In a vacuum, the idea to stop cut-and-clear racists and trolls from using Lemmy is not something that’s too controversial. Sure, they are being hard asses about changing their mind and allowing instance owners to configure it themselves (and I’m glad they changed their mind). But there’s a big overlap between passionate and opinionated people, so they have to be at times to ensure a project doesn’t devolve into something they can’t put your passion into anymore.
Second: I mean… what do you expect? In the issue above they actively encourage people to make their own fork of Lemmy and run that if they don’t like something from the base version of Lemmy, so I kind of would assume they do as they preach. Instance owners also have the option to block communities without defederation. Lemmy.ml is basically their home instance. If anything this is a reason not to make an account on lemmy.ml, but as long as that doesn’t leak into the source code of Lemmy, who cares?