This is what bothers me too. I think that it will never be so populated as it used to be but there is a way to motivate players to do it. As someone already said - new crafting materials in coil + mounts from extreme primals (plus they could implement some weekly quest for vanity stuff - killing all EMs like with titan, garuda and ifrit). I think they should synth all old em primals and coil to be like... i130 and lower and make it viable even in heavensward.