It's mostly due to the nature of FFXIV being an ongoing story that they're making up as they go. I imagine that the methods they employ are usually flagrantly destructive toward life because the story tells us that the aether in souls burns brightest at death, and I expect that would act as a catalyst for the reactions they're setting off on the Source to cause the aetherial draw-in.

I don't remember the details, nor where it's at in the story, but the 13th shard is The Void, and it was their first attempt at a Rejoining. The way I remember it is, they didn't prep or cause a Source side calamity, merely opting to drown the 13th in Darkness. Whoopsie doopsie they have to cause a big bang on The Source too.

In Shadowbringers they basically start giving us information that we've only ever had the cliff notes for even at their most explained back in 3.5. The lead-in for Shadowbringers is basically the narrative equivalent of click-bait. The goals of neither side change, merely the tools they're using to see those goals to fruition. Namely, Light and Darkness. The idea of Shadowbringers is to make you ask a question very similar to what you phrase in the OP. What is the true nature of Light and Dark? It's very much a running theme that tendencies or stereotypes are assigned by people, because that's human(or catboi) nature. It also goes on to show why the default settings for good are generally Light and the default settings for evil are generally Dark. You can still be a player on the board for either side, in spite of changing your settings.

So just keep swimming upstream. You'll get more info as you go, and it may satisfy you. If it doesn't, come back and start some more discussion!