I'd figured it was a few aspects of each of these, at first; primarily that the primal was from a previous era and those who summoned it were lost.
Buuut...Bingo! Ferne helped narrow it down a little bit. The title "elder" is bestowed on primals based on how long it's been since worship of them began. (So far,) everything else still applies, including the need for worshipers if the primal yet lives (which is why Belias' essence was slumbering). The explanation was essentially Fernehalwes and loremaster Banri Oda (who really needs a Voidsent alter ego) drumming up excitement for the 2.4/2.5 patch cycle by saying, "So, that means there must still be some worshipers around... Hmmm! Final Coil? Odin? Please look forward to it!"
When it comes to the primalsplosion, I've got more "Probably nots" than "probablys," unfortunately. We'll get there, though!