KNotifications has proper support for all major platforms (freedesktop, windows, macOS, Android). The KPassivePopup fallback is only relevant for Linux when no FDO notification daemon is running. D26604 addresses the corner case of a crashing notification daemon. IMO the case of the user having no notification daemon at all is not worth supporting since the users intention is verly clearly to not have notifications.
This allows to drop a good amount of code. It reduces the dependency on Widgets so we can eventually get rid of it which is nice for Android. Furthermore the reduced complexity will make it easier to implement our plans for KF6