My use of Apple apps has been dropping off, but so gradually that it didn’t strike me till you mentioned it.
But I think what really has changed is my approach to apps. Whereas I once used to use certain Apple apps exclusively, I now use different apps for one area. Horses for courses, sort of thing. Like for photos, I previously used the stock camera app and Photos (earlier known as Camera Roll) exclusively. Now I use Photos for quick edits, Notes for scans, Photoscan for scanning photos, Camera+ for macros, Snapseed for a bit more complicated edits, and Google Photos as my searchable online photo library.
The main reason why most people use Apple apps is because they are built-in, and work seamlessly with iOS. This is why I too use apps like Mail, Safari, Music (in tandem with the Gmail, Chrome and Google Play Music apps) as well as iBooks.
But there are other reasons. I don’t have any patience for the obnoxious ads that once used to rule the net. So when Apple allowed adblockers, I gave Safari a second chance as I had switched to Chrome at that point. Safari does have positives like the built-in password system, and its reading mode that makes thing readable on sites with tiny text, while also getting rid of any stray ads that survived the ad-blockers. Chrome probably does have similar tools. I do recall Chrome being able to pull off the Safari trick of showing me what pages are open on Safari in my other devices. But I have not felt the need to completely switch to Chrome as Safari is doing its job reasonably well.
Besides the above apps, I use the Clock app because it’s just a swipe away, Notes because it syncs between my Mac and iPhone (and the excellent ‘scan’ function in iOS 11), and Pages because I use it on my Mac. That’s not really much.
The other side of the picture is though Apple apps are losing out to apps from elsewhere, I don’t think Apple is too worried about it. Steve Jobs initially didn’t allow third party apps but later reversed his position. He realized that if someone else can make a better app for iOS, then it improves the iOS experience and that rubs off on Apple, and iPhone sales. Not to mention the multibillion dollar income from its App Store.