How many programmers did Microsoft need for 1 million OS installs vs 1 billion? The reason software can be such a money maker is that the marginal unit costs are close to zero.
I don't know the answer to either of those questions. But I don't think the economics of desktop OS distribution have changed all that much in the last 30 years. I'd imagine the marginal unit costs have gone from very cheap to basically nothing now that physical media is a rarity. If modern OSes take more development effort (which seems likely), I don't think that changes the economics.