Paying customers hire developers, and developers always, always create pressure within their organizations to use the tools and software that the developers are most familiar with. A few developers in this thread have mentioned some things they've struggled with; I've spent a lot of hours reading Phabricator documentation, sussing out Conduit, trying to learn how to use the whole system well, and there are still lots of gaps in my knowledge. When Phabricator becomes too different from the workflows that developers are accustomed to elsewhere, and it isn't immediately obvious that those differences make it better than the alternatives, then they're going to pressure their organizations to not use Phabricator.
Developers furthermore are always looking for ways to build their resumé. The further away Phabricator falls from being used in big, well-known, popular software projects, the more it becomes a hindrance to resumé-building. A developer can add GitHub or GitLab familiarity to their resumé for extra credit, and GitHub public profiles are a good way to say, "Hey look, I write and ship code on a regular basis". If I'm trying to get hired, and I want to learn about integrating CI, I might have to choose between GitHub Actions or Harbormaster. One of those is far, far more valuable for resumé-building than the other.
I can't argue with your focus on paying customers, but if your developer community isn't big and healthy, your paying customers will eventually not be, either.