probably not a huge deal, but bigger implications

> And what's the next step ? Drop support for /etc/passwd and /etc/shadow

Lots of places already did. Did you ever try to edit /etc/passwd when the name displayed by your company's Active Directory server was wrong?

systemd's user info server isn't yet another huge thing you need tooling for. It's mainly an aggregator. You can plug a parser for /etc/passwd+shadow into it, or AD, or whichever other service you want. It's a protocol, not an API or (worse) a file format from the 70s/90s that can't evolve and can't be extended. There's strict separation of concerns (in contrast to libpam and related ugliness).