What aligments would future calamities even have had?

[Iscah]

IIRC in the cutscene where Exarch explains history of Calamities and how they happen, he mentions that once amount of rejoined shards is more than unrejoined, the balance will tip and all the rest will rapidly devolve into rejoining as well.

I don't recall anything being said to that effect. I suspect you are getting confused with the explanation of an individual Rejoining starting as a trickle of aether to the Source before the triggering event for the associated calamity cracks it fully open.