New iPhones and DPI
In Adopting Adaptive, Pablo Bendersky considers whether or not new iPhones would have a higher DPI.
One possibility for the higher-DPI case:
If Apple wants to have the text look the same size, they might tune the Dynamic Text default sizes on the new device to compensate for the higher DPI.
This could account for why Dynamic Text has been so strongly encouraged.
Also from Pablo:
There are some apps that rely a lot on custom fonts, such as Vesper. Since Dynamic Text does not allow you to use custom fonts, I wonder how those apps will work around a different Dynamic Text default size. There are tons of valid workarounds, like fetching the system font size and base your custom font size on it, but an Apple blessed solution would do a lot to help in this direction.