There are now more Apple products that are likely to get a yearly update than there have ever been before. 3 lines of iPads, two lines of iPhones, and the Apple Watch. Apple would be insane to update them all at the same time each year. For one thing, businesses like to show profit in all 4 quarters, and it's hard to do that if you concentrate all your sales into one quarter. Second, even hardcore Apple fans don't upgrade all their devices yearly, and most don't upgrade all their devices at the same time, so Apple would be cannibalizing their own sales. If I have $1,000 to spend on an iPad in April, that doesn't mean I'll necessarily have $1,800 to spend on an iPad AND an iPhone in October.
The iPad Pro and Apple Watch both throw a monkey wrench into the predictability of the upgrade cycles. I think we'll start to see things becoming staggered between all these different lines, except for the iPhone which will remain around September because it's their biggest moneymaker and they know a lot of people are on financing plans or in contracts so will become eligible for an upgrade around the same time of the year.
But it would be wise of Apple to retain a spring and fall keynote and to stagger their product releases.