Demons have the potential to be a very potent synergy. Mana burn, dealing true damage can absolutely devastate any comp that relies on spell casters, namely sorcerers, but even glacial can get flattened by it. The problem though is that Demons both cost way to much, meaning they show up late game, many of the individual demons are rather weak, and it requires a strong economy to even start; let alone build up.
And what do you get if you do manage to 6 stack demons? 70% mana burn? Sure that is pretty solid in a void, but for the amount of investment needed to get it going and how weak your formation would be, it's simply not worth the investment most of the time. Now both of these options shouldn't be taken, as that'd make Demon WAY to good, but one or the other would help make the investment easier, or more worthwhile.
I've seen someone suggest Varus and Vayne should have switched cost, and I agree with this, though I also think Eve and Aatrox should be both reduced from 3 to 2 gold. Both simply aren't strong enough to merit being 3 gold each. Even when I'm trying to go Demons, Aatrox simply isn't bulky enough to fulfill his role, if he's up front he'll be dropped likely before he finishes casting his ult, if he's on the side, he won't be dealing damage fast enough to fold the enemy line before a different character actually causes either team to fold. At which point, he falls over due to lack of bulk. Eve's ult greatly hurts her dps. Sure it's a great execute for taking out squishy carries, but you'll rarely level her up to the point she can take on a carry the opponent has invested in (gold Vayne for example), at that point, her ult just disengages her, which can be good for dropping aggro, but also drops her DPS till she gets into range.
Demon 1 should stay the same, as getting 2 demons isn't that hard, and should scale through to 100% at Demon 3. Getting 6 demons is both REALLY hard, due to 2 of them being 4 and 5 gold (Brand and Swain respectively). Yes you can make someone Demon with spatula and tear; but both of those items are really good components for other items. Tear builds many of the best items for carries, both AA and spell carries. Spatula builds the super powerful FoN, as well as letting you build into other origins and classes that are better; Glacial, Assassin, Blademaster, ect. So someone investing to make a piece Demon should be rewarded.