I think they're holding out hope that they can continue to grow revenue through advances in adtech when their subscriber numbers hit the inevitable plateau. Eve

I think they're holding out hope that they can continue to grow revenue through advances in adtech when their subscriber numbers hit the inevitable plateau. Even offering an ad-free subscription option would eat into their ad platform, and Netflix has recently demonstrated how tough it is to raise the price of a subscription (or expire a years-old discounted rate). With ads, they can at least hope to grow per-user revenue in the future.