With its $800 iPhones and $1,400 Hermès smart watches, Apple’s not exactly in the business of bargain-bin pricing. But the surprisingly low-priced debut of Apple TV Plus, at $4.99 per month for a family subscription, is a signal that the streaming video market of tomorrow — soon to be awash with services from Disney, WarnerMedia and others — will be one with pressured margins and claws-out competition for viewers’ wallets.