Especially bundling these together as equally bad/legacy is weird. LDAP itself can actually be quite easy to understand and work with if you limit yourself to a useful subset of functionality, ie. a account/group directory. Any Kerberos compatibilty/design inspiration should IMO be avoided though, as it was designed to not use public key crypto, and that translates to a lot of its weirdness.
What I’d like to see is one of these modern offerings actually expose an LDAP facade (bonus points for translating app-specific passwords into binds and for flattening nested group membership) so that it’s easy to bridge existing software which expects LDAP into this newfangled web-centric world. Things like an email MTA/MDA, a PHP app that wants a user directory or even nss_ldap for unified UIDs/GIDs across machines.