There are limited humans on this earth and if you consider, that you cannot reach them all and there is also competition, which realistically also takes some of the market, why not be fine with reality and a saturated market you cater for well?
That is in theory. I think most businesses lose the original vision at some point though and their products worsen, instead of improving. I would guess, that it is not in Netflix' genes to keep one core product and vision of the product.
The idea to show ads on Netflix is like the nail in the coffin. Users would jump to the first capable alternative, that does not show ads. Netflix would inflict itself a weak point, at which competitors could jump in and start eating their market share. It would be a big worsening of their product. Users might think: "I am paying for this, yet I am seeing ads?!"