More: Responding to Ira, wouldn't it be unusual for Apple to abandon important OS functionality in favor of third party apps when their usual MO is to incorporate worthwhile 3rd party functionality into the OS?
Apple did a study of how their iPhones are used and found that 60% of iPhone usage is for texting. That does not mean the remaining 40% is telephone usage; there is e-mail, Internet, etc. I take this to mean that (surprisingly) the iPhone is used as a phone less than 40% of the time. Consequently, ringtones are not critical, but flashier emojis and general texting capabilities are. Witness the new (as of iOS 10?) within Messages app App Store where you can get animated GIFs, stylized text, etc.
So just how important and worthwhile
customized ringtones are to Apple and OS functionality may be "not very". At least from Apple's perspective.
I too have customized ringtones, but so far they have not been lost.
