Recently Apple added (and actively encourages) the ability for developers to upload bytecode to the App Store instead of ARM binaries so Apple can more easily dynamically recompile for new architectures and optimisations. Of course bytecode is considerably easier to revert back into readable source-code (especially as Swift/ObjectiveC retain (some) symbol names in compiled output) - so it's not outside the realm of possibility that an unscrupulous Apple team is disassembling cool apps to see how they work then re-implement them for the next release of iOS.
Can you identify a type of app for which reverse engineering it would be easier than writing their own? Software is usually easier to write than to read. If an app has such a magic secret sauce, and it's of value, then it should be protected by patent or copyright anyway.
An example that comes to mind is a high speed image compression app for taking rapid sequences of photos. Apple bought the company or the rights so they could include it themselves.
In my experience, it's always been easier for me to implement something once I've seen a working example of it. That's basically what examples are for: A "cheat sheet" for reverse engineers.
Software is only easier to write than read if you have an idea what it's supposed to do. If you've ever googled "how do I do X?", then you likely have reverse engineered the answer you found to fit your particular use case.
In addition, and in some countries, you can't patent software (thankfully), and so innovation comes through reverse engineering naturally.
I really don't think Apple, with it's war chest, is actively disassembling code to steal it. As has been demonstrated time and time again, they will just buy companies that have awesome tech and IP. Far easier.
