i don’t speak for the staff either, but i can imagine some reasons for adopting a new ID system in the future, though i would think that if they adopted a new ID system in the future, they would do it in a way that is as backward compatible as possible, where most of the time, you would be able to get back to observations originally created in the current ID system.
i think if you needed absolute certainty that IDs would not change, you might consider an alternative, such as described here: https://forum.inaturalist.org/t/incremental-counter-in-a-custom-field/11892.