My agreement with your conclusion is 50/50. Yes technically Emigration/Immigration is a “permanent” state, so you could be correct on that account. However, historically once someone spends everything they have to move to a new country they rarely go back even for a visit. It was not until the 1900s that people could more likely return for visits.
So I believe that unless the move was designated as a visit, most people really thought that they were leaving for good because they probably sold everything they had to make the move and should be designated as Emigration/Immigration. I have friends that moved to the USA, lived there for 40 years as citizens, then returned to Norway at retirement. They would not be Arrived/Departed.
Also as an a side, GEDCOM support Immigration/Emigration, my twisted thoughts about Ancestry.com is that they did not use Immigration/Emigration because either the programmers did not know the difference or they were afraid that their user base would be confused!
But I do see you point about “moving globally, is not always permanent”!