What problem does this solve or what need does it fill?

Updating the app state is a very flexible and expressive operation, which is fundamentally deferred. States typically changes only once per frame, after PostUpdate.

This can lead to unwanted delays when handling multiple dependent transitions at once.

What solution would you like?

As discussed in #15127, it would be nice to remove this meta-schedule, and instead run the various OnEnter / OnExit / OnTransition schedules on demand, leveraging the new observers. This is a good fit for "highly complex rarely exercised logic", and resolves the delay problem mentioned above while simplifying the mental model for consumers.

#15127 should be tackled in concert with this, removing NextState completely, and relying entirely on commands to handle transitions.

What alternative(s) have you considered?

Users can manually add more state transition systems to their schedule, including in their own schedules that run inside of Main. This is somewhat messy, nonstandard and hard to discover.

Additional context

Discussed briefly on Discord with @MiniaczQ here.

This design was not originally taken (including in the substates refactor) because observers didn't exist at the time!