Honestly, Apple is going to gain more in the Mac user base this year than they have in their entire history. So I don't see them going downward from here.
Teenagers have more choice now (when I was in high school, you either had Windows Mobile, Palm, or the just introduced iPhone 2G), and obviously because they now have phones that do everything, a standalone iPod isn't needed. I think if we didn't have a huge smartphone market, the iPod would still be a huge thing like it was in 2004.
And as for tablets... The thing is that you can easily keep your iPad for a couple of years and be fine with it as long as you don't drop it or something malfunctions - I never replace my Apple tech until it breaks. Everyone praises the price of the Kindle Fire, but then I can pick out at least 50 people in the forums I go on (including here) that upgraded the day the HD was announced.
The people who are spending less money on their tablets are suffering more because of things like not getting the latest version of Android, not having enough RAM, and apps and games not working correctly. Those that own Kindle Fires are constantly having to side load stuff, are trying to get rid of the carousel, and make things like the basic Google Maps work for them. And the Fires are now littered with ads everywhere, and the web browser is just a way for Amazon to get insight on your usage.
Why pay $200 every year for a new tablet when you can buy one for $350 and have it for a few years with updates at least three times in a year?
And a bunch of people complain, because it's not the iPad experience. But it's not really Apple's fault that people are too cheap to spend money on quality pieces of hardware, and instead want to throw it away on coins in games and fast food. They can't do anything about it though.