But it's not an "European mobile operating system", it's a “deGoogled” version of Android OS.
How can Android OS challenge Android OS?
Privacy-respecting phones already exist in the US market (I think it's pretty much illegal in Europe to get cell service without government ID, but that's not firsthand). If you think you care, buy https://inteltechniques.com/book7a.html, get a Pixel phone, and try for yourself.
Ironically, they're handicapped by Google's SoCs being quite bad (though seemingly not much worse than most recent Qualcomm flagship products, this most recent generation excepted).
Edit: this site suggests that there are quite a few countries in Europe where you don't have to register: https://prepaid-data-sim-card.fandom.com/wiki/Registration_P...
/e/ OS already works, I've been using it for 2 years flawlessly.
What's the difference with GrapheneOS? They target a slightly different audience. GrapheneOS is really into security, and for that they only target Pixel phones. CalyxOS is based on Graphene and tries to make it more user-friendly (at the cost of being less secure: different users, different threat models). /e/ OS is based on Lineage (I believe), and also tries to make it more user-friendly.
They all have a focus on privacy (as in, not being tracked by Google & Co), but they do it differently. I would argue that they are all on the same side.
so it seems that /e/OS implements less security features than grapheneOS.
i haven't used grapheneOS so i don't know if there are any other differences.
/e/OS certainly is more userfriendly (compared to the lineageOS it is based on)