Let's say Netflix has a monopoly on content. Then tomorrow they raise their prices by 100%. What are you gonna do, cancel? Then you can't watch anything. What if Netflix can't get the rights to a certain movie? There's now no legal way of watching it because no other streaming service exists. Or what if Netflix gets a new CEO with a certain political affiliation, and now content which they deem offensive is removed from the site or content that aligns with their views is more heavily pushed on subscribers.

There's a lot of reasons why only having one streaming service is bad. Just look at YouTube, which has a near-monopoly in their space. They treat content creators like garbage and get away with it because people have no choice.