If Tim Cook cared about making the naming schemes coherent, there would have been an iPhone 9.
Apple has had a huge number of opportunities to bring back coherency to their product naming conventions. They have passed nearly every single one up. I think Steve Jobs was so disgusted with the alphanumeric soup that Mac models had become in the '90s that he decreed that nomenclature was to be as simple as possible. I’m actually surprised that iPhones ever had numbers instead of being “iPhone (year whatever)”.
It wasn’t until last year that they finally realized that “OS X 10.blah”/“macOS 10.blah” was sounding more and more awkward every year and moved on to 11. They should have done that when they changed the code name scheme from cats to California landmarks—it was the perfect time. (Of course, I also think 10.9 should have been code named “Serval”, just to see Apple Store employees trip their tongues trying to say “OS X 10.9 Serval Server”.)
Besides, I think all the fuss about “iPhone 13” is missing a big point: Apple has not shied away from “13” in version numbers in the past, and has shown no indication that they will this time. We had iOS 13. We had macOS 10.13. Heck, we’ve had 13-inch MacBooks since 2006. The world did not end. The Apple-focused media barely even blinked.
I think that if Apple decides not to use “iPhone 13” for this year’s phone, the most probable course would be to make it the iPhone 14. It’s the obvious choice, given Apple’s past behavior.
What it almost certainly will not be is “iPhone 21”. If they go with the year, they’ll likely stick with the format they’ve used for every other product whose model name contains a year, and call it “iPhone (2021)” or “iPhone (2022)”.