Either laws work the way they're written and 4 members of the court disagree or they work the way the Supreme Court says they work and a worse ruling could threaten those citizens.

That was very much on the table, though not locked in.

This wouldn't really be applying laws retroactively, but deciding what the law has always meant. If the decided meaning is that children of illegal immigrants are not automatically citizens, that means they weren't automatically citizens beforehand. Thus there wouldn't really be anything to revoke, they just never were citizens in the first place.

The executive order that prompted this was only aimed at babies born after it went into effect, but I see no reason it would have to be that way.

That's not how we have traditionally thought citizenship works, but that is exactly what the Trump administration would've gone for next if the supreme court had ruled in their favor in this instance, thereby setting up the _next_ supreme court case.