Apple is going exactly the way it did with the PC market in the 80s and 90s. They start with a great product that virtually creates a market to itself. High prices and margins result.
Pretty soon, they have 80-90% of this new market, especially in the US, and are creaming it in.
But their selfishness in wanting to keep everything for themselves - OS, hardware, even peripherals and service/support means that other vendors have to find a way to compete.
An alternative OS that is available to all gets picked up, and dozens of manufacturers pile in making hardware. It may not start out anywhere near as good, but the sheer volume ensures it catches up pretty quick. Soon you get equivalent performance and features for a fraction of the price of Apple. Meanwhile, Apple chooses to retain its margins and profits, at the expense of market share.
Already in Europe, Apple is down to maybe 10% of the market for smart phones. Android has the rest. They have more in the US, but even there, the same economics will ultimately prevail. Apple beancounters are too busy looking at the high profit margins they still have to notice that some apps and developers are now not bothering with Apple, because Android is 90% of the market.
Pretty soon Apple is a niche product, most software, and virtually all new innovative software doesn't support Apple devices, and their business rapidly starts to collapse.
If Apple still had Steve Jobs, you might think he'd do it again and find a whole new segment for Apple to dominate and milk for 10-20 years. But Cook? I doubt it.
I know people have been predicting Apple's demise for ages. But looking at their phones now, they're not really leading the way any more. This big screen things are copying Android... Apple spent years resisting it and insisting their small screens were the optimal size. And the big kicker for them is declining market share. It's hard to see a long term future if your whole ecosystem is less than 10% of the market. Just look at what Windows did to Apple.