... which diminishes the likelihood volunteers will join your project, because now they have to go learn another site.
If you know the basic functionality of Github, do you really have to learn to use similar functionality of Gitlab, Gitea, etc? Is it not enough to be familiar with the concepts?
Just like when people were switching from the blue e to firefox/chrome: these were different browsers, with different UI, but the concept of browsing the internet was the same. So in the end, the different UI didn't matter.