Apples downfall could be software, especially AI. Google/MS/Amazon or even Facebook all have incredible distributed software systems, lots of AI R&D etc. while Apples attempts are somewhat lackluster.
I don't think it will be there downfall, all of those companies have failed to make compelling mass market hardware and have shown no lack of willingness to write software for iOS. Whatever they do in AI that benefits the consumer, they will make sure it runs on iOS.
If Apple gets so far behind that they need to offer it to their customers, they will probably make it easier to integrate Amazon, Microsoft, or Facebook's offerings.
Google could probably sell a lot of Nexus phones if they really tried. They'd probably burn their bridges with the other Android vendors if they did this though so it would require a pretty significant strategic shift.
