This concerns me as a GrapeneOS user.
No. It is not. Lineage and a lot of others are major players.
They just have different focus. GrapheneOS cares about being secure. Others cares about being, well, custom, and "privacy", "privacy" as in "noooooooooo I don't want to install google stuff jfc", instead of being secure against random threat actors.
The Pixel 6 I have will be hitting its end of life in October 2026. I had wished to be able to simply get a newer Pixel and just carry on.
LineageOS is the broad-support Android distribution. Its current version supports all Pixel devices. Third parties have even built recent-ish versions for ancient devices (e.g. Android 13 on the Nexus 5).
The google pixel line were the only smartphone you could buy new on a brick and mortar shop on monday almost anywhere in the world and have it running on a custom rom on tuesday and graphene was leading the way in pushing privacy and security at its maximum while other rom maker are making a lot more concessions.
LineageOS still is king to save old devices but grapheneos definitely is a major player because some people like me did chose second hand google pixel with the sole purpose of being able to run graphene.
Ironically, running a google smartphone was the best way to live a life without sending data and rely on google services.