According to this article, all of Apple's wealth and revenue comes from the iPhone; over 70%.
This means that Apple is now a phone company with some computing stuff on the side. One big mistake, and people could switch away from the iPhone. It's much, much easier to switch phones than to switch computers/operating systems.
You can see how fast everything went downhill for Nokia and Blackberry. Nokia is very big outside the phone world (in networking and industrial applications and such), but Apple isn't, AFAICT. If the iPhone fails, Apple fails.
If Windows 10 fails, Microsoft will possibly fail as well.
Then we have only Google and it will turn into Skynet.