"I wonder... are there enough Muggleborn babies that they are unable to smuggle out for one reason or another that the list still includes many Muggleborns? Or does Voldemort think that there are fewer and fewer Muggleborn babies being born, because so many are left off the forged list? And if he does, does he take that as evidence that his plan is working, and that he is stopping Muggles from stealing magic finally?"

I would posit that the decrease in Muggle population would be all the explanation he needs. We've already established that the population maths are fuzzy in this world, so that would seem like a sufficient explanation to me.