About the Echo (Spoilers)

[Eyvind]

The thing is, there are plenty of people being awoken to the Echo all the time, they just don't understand what it is and so, out of shame or horror keep it to themselves (this was specifically mentioned back in 1.0 that those being awoken to the Echo would often face persecution and harrassment by others simply because they did not understand what the Echo really was - hence why Minfilia ran the Scions' predecessor organization the Path of the Twelve, it was a place of sanctuary for those awoken to the Echo and also show the mundanes of Eorzea that Echo users were not to be feared by sending members out on missions that would benefit the citystates). Cases of the Echo being awoken in people seem to coincide with the lead up to a Calamity however, but again why that should be is unclear.

As for the sky shower seen at the beginning when the player is awoken to the Echo, it is still not known exactly what that is meant to be represent, other than it was there even in 1.0 (long before SHB's story revelations were even thought of).

Now I'm thinking

1) if people are being awoken to the Echo all the time, why is there not much mention of it in anything dealing with the past...that is, we learn plenty about the 3rd and 5th astral eras, but rarely about folks with the Echo
2) Echo use tends to awake with Calamities
3) We're on the Source, and Calamities are when people get rejoined. The average Eorzean is now 8/14 rejoined

If the Echo is an Echo because it's an echo of when people were Amaurotines, then it might be that we can "hear" that echo much more effectively the more we're rejoined. I remember Elidibus saying that we were becoming more powerful in its use, and he was hoping to recruit us saying we would be "of one mind" with the Ascians. He probably thinks that as we're rejoined, our Echo will grow stronger, and we'll begin to remember what it was like to be like them.

If that's true, it would mean that each Calamity makes echo users more and more common.

As for the original question, I think if I might rephrase it a bit, if Echo users are descended from shards of sundered Amaurotines, what percentage is in that category. Alphinaud explains when it comes to the pixies that most souls dissolve back into lifestream, but a few "strong souls" remain intact. Anyone from Amaurot and reincarnating probably fits into the latter category, so the question becomes what percentage of sentient creatures have souls that came into being since then? Could we even guess without more information...? And do these souls get stronger as rejoinings occur, perhaps fusing with other new souls on the rejoined worlds, or remain the same?

EDIT: Jossed, at least for the two categories of souls. Clearly Banri Oda was reading this post

It seems everyone - for that matter, everything alive - on the Source is getting rejoined, not just a few select souls. So sayeth Banri Oda just today, who writes this game's lore. So its aether is increasing which increases its "potential" power (emphasis on potential - not everyone reaches level cap).

That raises the question of whether there are new souls at all...up until now, souls get dissolved in the lifestream except a few "strong" ones. Is that wrong, an in-universe mistake...? Or are we intact old souls getting our old pieces back, while new souls are getting random infusions of unrelated aether?