But AppleTV isn't. There's no lock-in for Roku or Fire TV... You can't buy anything from Roku, and Amazon has apps out for all TV boxes.
Apple is Sony here, one company that won't allow interoperability. Just like betamax, it often has superior specifications, but won't let anyone else play in it's walled garden.
Apple might sell more than any other single competitor, but it's still losing the war. The brand is built on "quality costs more", which is great for companies that don't need to interoperate, like watches and knives and even cars... It can't compete on price because it will hurt the brand... So they're stuck making more money than they can spend, while losing the war.
Not a bad place to be in, better than betamax, but their tech will suffer the same fate, unless they open their walled garden.