Are shards different?

[Cleretic]

The writing surrounding aetheric density doesn't feel very consistent. The Ancients are established as having their extremely long lifespans, creation magic, and proportionally immense size on account of it, but despite having over 50% of their density, people on the Source are interchangeable from those on the shards. For a while, I thought the explanation might just be that rejoinings only increase the overall quantity of aether on the source and don't actually make the existing people more "complete", but then there was that line in 5.1 about how exceptionally aetherically dense the Scions are compared to the denizens of the First. But then... They're still able to use dynamis, regardless? And then you have that comment from Yoshi-P about how reborn Ancients will probably just have some extra special powers but otherwise be normal, and it's like. What was making them immortal, then, exactly?

It feels like the writers were going for more of a rough vibe off-the-cuff than something meant to be mechanically analyzed.

I mean, you should answer this one in a more figurative and allegorical sense above all; the Ancients are clearly intended to be godlike figures, so they have godlike physical qualities that disappeared once the Sundering happened and they weren't gods anymore. Similarly, we're not gods among inferior beings in Shadowbringers because that's just not the story they were telling. So any explanation kinda has to work backwards from those points; the story must work first and foremost, so whatever explanation we come up with must therefore allow both of those stories to still work first and foremost.

So with that, I think the best explanation I've got is that the Ancients were just literally a different species entirely, living in a completely different environment. Remember that their model is unlike any in the present world; they've got the 'hyur head, elezen frame' that the Garleans have, but without the third eye. (Also, they're enormous, there's that too.) Also keep in mind that even in the present world, lifespans range pretty hard; it's pretty well-known that elezen lifespan is 120 years and viera are in the 200 range, but as I recall miqo'te are the low end of the range at about 60. So there's not really anything dissuading the reading that they were just a different, now-extinct species. Perhaps one that's not really capable of being sustained in the less aetherically dense environment, so they gradually either died and were replaced by, as mentioned in a developer Q&A, sundered souls desperately yearning for aspects they either remembered having or want.