As someone who relies on systemd, but wants to have alternatives:

While it is good othat other login managers will still be able to start Plasma, making the default new login manager reliant on systemd is bad. It means that non-standard installations of KDE will now require more manual labor to make it work just right. And while installing sddm is not big of a hassle, this sets a precedent that can later be expanded, making it a death by a thousand cuts for everything that dares not use systemd in its operation.