I guarantee 99/100 humans on this forum either currently host with AWS/GCP/Azure or have worked at a shop that does. And I bet an outsized portion of those AWS/GCP shops also host on Azure for Azure AD.
There is no one that is ready for a de-Microsofted world. Even Linux distros have been increasing their support for integrating into the MS ecosystem and forsaking alternatives because how prevalent AD is. Even the most prominent alternative FreeIPA is designed to compliment an AD installation, not replace it. The best supported directory/central login server on Linux is AD.