Earlier quoted context omitted.
> Is AD even worthwhile anymore for a new company? Probably but Samba already does AD so you don't need Microsoft to do it. I guess PolKit at some point may offer similar functionality as group policy.
I don't think so. Start with Azure AD if you want the Microsoft system.
Having said that, as a former directory engineer (iPlanet/Sun/Oracle/ForgeRock) I don't think any of the Azure AD workarounds, including samba's (who really needs CIFS in a world where files can be served up over https with secure OAuth?), are worth all the extra effort. If you need an enterprise directory, you should deploy one. The good news is that both Ubuntu and Red Hat now support Azure AD, so you're not stuck with half measures.
Of course not every shop _needs_ a system/network directory, and both those Linux ecosystems support a range of user and system management options that can do the job. Even if you finally find yourself in need of something more, AWS and GCP offer competitive identity services that can work just as well with non-legacy systems as Azure AD (so long as you don't have any Microsoft PaaS or SaaS dependencies).