I think you're right. 5 mana draw a card or 5 mana make, at best, 2 2/2s isn't ideal. Interesting with phyrexian mana but how often will you be able to play this and another spell in the same turn.
If there is support in set for paying life as a cost, maybe this sees play? But it'd be a lot for the meta to allow a slow PW in a suicide black style deck that also wins by milling.
Well, thinking about this more, there's a slim chance it could combo kill. You'd need two things. First, Vorinclex, Monsteous Raider in play. Second, a repeatable source of paying life. Something like an expensive enchantment with "pay X life: surveil 1". It's be a three card combo, but you could drop Ashiok, exile your library until you have mana value in exile equal to or greater than your opponents library size, then ult, mill them out, and they die on draw step.
That said, that's a lot to ask for. And it's pretty unlikely we'd get an abusable life paying outlet. You could also do it with some sort of "pay any amount of life" but that's even more unlikely in standard.