No need to find quotes, I recognize all that info. Eh, I think you're right about where the initial Splinters to fuel Seons came from. But Seons can expend the Aons in their core and die. I think they've been around so long that they would have gone extinct if that was the case. And the other "natural" Splinters we've seen, spren and the Divine Breath, are not themselves sentient. I think it probably requires an interaction with a human or some other intelligence to bind the power to the Aon at a Seon's core, which I personally believe forms the foundation for their mind and personality.
The argument could be made that the reason there are so few Seons is that none have been made since the beginning, but I find the abrupt drop-off in population intriguing, even if I factor in the fact that Seons are now unfashionable. I think they were being made, but something's stopped it now. This is about as baseless as speculation comes though, so I guess we'll have to see. I fully expect to have this blown out of the water come Elantris 2.