Here is how I can predict further decline, I won't be upgrading this year or next either. 2021 will be a deciding factor, might just keep using my 2020 X or switch to a cheaper Android and Windows. The folks who upgrade their iPhones and Apple devices like Macs and iPads every years are very niche; probably like 0.5%.
When I walk around the city and use public transportation, I see more Touch ID iPhones than Face ID. Users only upgrade if its extremely slow, broken or they are sometimes changing to another cellular carrier. I was in a store the other day and a guy came in wanting to upgrade his iPhone 6, which has 64 GBs of storage. He said he wanted to upgrade because he is running low on space. When the store agent checked, he used just 32 GBs of the 64 GBs. He still wanted to upgrade.
He looked at all the line up and decided to go with the iPhone 7. So, he skipped 2 generations of iPhones, even the iPhone X, which he could have gotten for a good deal. So, that says a lot about taste. Users remain comfortable with Touch ID. One silverlining for Apple is loyalty. The store agent tried to sell him on a Samsung S10 and he said no, and it was thanks to the fact that he uses a Mac at home.
With how much Apple is messing up the Mac lately, once users change out to Windows, they will have a lot to worry about. Because its at that point users will just give up on Apple all together. The next version of macOS is going into weird direction and I am not enthused with that popular developer Steve Troughton-Smiths enthusiasm about it - he is iOS obssesed. There are some things iOS does good, but I don't think its apps are good enough as desktop replacements.