Maybe also too optimistic as well.
Let me start by telling you a story about me trying to buy clothes for the first time:
Back then, I have no idea of branding and the symbolism behind, I don't even have the idea of quality.
After many hours of browsing and comparing, I picked what I liked based on showcase pictures and user reviews, completely brand-blindly.
A week later when they arrived, I quickly discovered that few of them are false advertisments, some of them are in the wrong size and one got damaged during shipping. And for the rest items that were "just right", half of them ripped/deformed after half-year of use.
So, as a normal person would do, I created a table which consist of factors such as durability, price and styles etc to help me increase my "success rate". And after few years of iteration, that table ended up points me to just few brands that happens to be well-known and big.
I eventually settled on those brands with membership cards etc, and now whenever I need new clothes, I open their store page.
What I learned from my experience is that discovery itself generates cost, and the cost is nonrefundable. It is reasonable for an user to settle on a trusted brand. It's not brand loyalty, it's just rational decision making.
For most users, choose GitHub is rational, because it's the most trusted platform out there, and it provides good services (including the infrastructures and socials). You already knew what to expect before you even sign on, that's the level of trust GitHub delivers.
It's really hard to ask users to discover and move to the alternatives once their needs are well-fulfilled by the current one, even when the alternative provides the same level of service.
What makes it even harder is that services such as BitBucket, GitLab, SourceHut, Gitea etc can't provide the same level of service that GitHub offers for free. At this circumstances, the megacorp such as GitHub will always win even when the user is pro-indies by default.
But we are not completely dead in the water here. GitHub succeed because they created something new at the time, something better than what SourceForge offered.
I assume if someone wants to beat GitHub, they need to do the same. Don't just create alternatives, create something new that also does old things better.