Yeah. To maintain the matter and energy density of the universe, Kyubey would have to keep up with expansion of the universe, which is already three times faster than the speed of light and is only increasing. The amount of energy he'd need to harvest will quickly skyrocket into infinity, and since grief seeds don't rip apart the time space continuum whenever they appear, they don't produce anywhere near the energy needed. So it's pretty much impossible for him to save the universe, no matter how much energy he harvests. But if he's only concerned with keeping his home galaxy burning, then he probably doesn't need to worry.
It's actually laughable that Homura's timelines would be able to produce enough energy to alter the universe itself, unless she's gone through nearly a gogleplex of loops.
Urobuchi has no sense of scale, like every other hack scifi writer before him.