How well are their competitors actually doing? Hulu gave away a year of their service for 99 cents a month on Black Friday and just dropped their regular prices, Netflix just raised their prices and are trialing even higher prices as well. Most streaming service exclusives are buried under complaints about having to subscribe to something that isn't Netflix to watch them[1]. People regularly decry being expected to subscribe to 4 or 5 different services to watch the shows they want to, despite the combined total still being less than traditional cable.
Netflix also has a huge first mover advantage, and the subscriber base and cash to move quickly to squash any entrant to the market. And they have a huge technical and infrastructure advantage. They alone make up about 30-35% of all US Internet traffic, and they've got their own CDN boxes all over the globe, hosted for free by the ISPs. Don't like their US subscriber numbers account for two out of every three households or something like that?
Their back catalog agreements with traditional studios are definitely their weak point though, as the studios are increasingly realizing that they need to charge more for access to their catalogs or claw them back to start their own services. Netflix has shifted heavily to their own original content, and in time, CBS, Warner, and Disney's sizeable back catalogs are definitely going to make some strides on their own services, but they'll be following, not leading the pack.
[1]This is half the complaints about Star Trek: Discovery in a nutshell. My guess is most people have come up with reasons they didn't like the show based on their initial upset about it being on a new paid service. Discovery's excellent, btw.