If nothing is to be changed on soulstone availability, then perhaps lowering the cost of the first 4 Maranian Arts (Oath of Faith, Dark Rituals, Maelstrom of Souls, and Bloodthirst) could be looked into. Reducing their scroll cost from 100 to 50, just as the higher Maranian Art scroll costs are, would at least help players gain access to those units before they're obsolete, though I doubt even that will be enough. 15-25 soulstones a month (maybe 25-40 a month for seasoned players on new servers) just isn't enough to get those 4 Maranian Arts before PFs and Huntresses, much less Wyverns.
Dark PFs make Seekers useless. Regular PFs, limited Amulets, and higher Maranian Arts also make them useless.
Huntresses have the same affect on Lost Ones.
Reiters could have minor use due to their speed, but limited Amulets, higher Maranian Arts, and severe offensive/defensive imbalances make training them not worth the cost of the reduced soulstone availablity. They don't even have reduced food consumption. Dark Nomads are half their cost.
Priestesses are useless the moment a player realizes BGs can easily turn Pikes, Paladins, and Knights into Wyverns.
The limited availability of soulstones make these units even more useless....from a cost-worth perspective.
You may understand this very well, but I'm not sure the devs do.