iPhone sales drive sales of their other products and services. Apple is already facing heavy competition to iTunes; overall sales are declining and app revenue is making up a larger % of iTunes revenue in recent years. The decline of iTunes revenue is largely attributed to the glut of streaming services. I suspect their competing streaming service will need to be a loss-leader in the future in order to fuel grow for their cash-cow.
Now, I don't think that Apple is dying, not at all. But I can see how their situation is a bit more precarious than a heavily diversified company like Microsoft (whose failures demonstrate their impressive resiliency).