That's the demand for GROWTH.

They've hit the limits of iPhone sales - and upgrade cycles are slowing. Hardware products in general are "streaky" - ie. demand and sales drop in the period after a new product is released, so how often can you produce a new version and what happens if that new version isn't a hit?

Whereas subscriptions provide recurring revenue. And services, in general, can bring in more money without an equivalent increase in costs.

I recently read "Apple in China" and one of the things I hadn't realised is how many people at Apple came from IBM under Tim Cook's reign. What he's done for Apple is turn them into a predictable, consistent, revenue machine.