Further insight into Earth/human ascension

[anagogy]

I mean how can 3rd density beings exist on a 4th density planet?

as far as I'm aware, the 3rd density sphere will stay in activation until the transitioning is complete.

This is my understanding as well.

In fact, even after the transition is "complete", there is still a 3rd density vibrational plane or "sphere".

It is just "in potentiation" or "not activated" in the sense that there will be no 3rd density entities inhabiting said plane. And also, in the sense that our 3rd density artifacts or constructs (buildings, structures, etc.) will be gradually broken down by nature.