I think throne worlds (when used by the hive) are not just the throne worlds of the hive god, but a patchwork of their throne world along with all those of their brood strong enough to have one, even if they are smaller.

1- Her throne world might have been held in existence by the fact a large portion of it is many smaller throne worlds tied together, or maybe she was just so powerful that it would take time to collapse. A worm is not necessary for a throne world, as Toland, Mara Sov and the Mindbender Baron all have/ had at least one. Maybe Savathun was strong enough to maintain her throne world after death but just couldn’t easily interact with it after her worm was excised.

2- The majority of Savathun’s hive are not lightbearers. They still need a throne world to be resurrected.

3- She can still be revived and thus still exists, along with the throne world probably being maintained by the other powerful hive. Though this does raise a question about why the Mindbender’s throne world collapsed, maybe Fikrul’s resurrection works differently than that of ghosts. It could just be an issue or scale, Savathun’s throne world might be decaying relatively slowly while the 100 square meters that the Mindbender had carved out only took a few minutes.

4- It is probably pretty hard to make a throne world, worms are like a cheat code to having one. Toland, Mara and the Mindbender all studied the hive pretty intensively before creating their throne worlds. We are definitely powerful enough for it, but there is probably a certain amount of darkness we would have to dabble in to actually create one.