If Apple can pull this off, I'm all for it as there are certainly some apps I use on my iPad or iPhone that I'd like to monkey around with on the desktop.
At the same time, this will be crushing for some developers who have iOS and macOS versions who have to make a choice ... do I make a truly universal app and raise the price or keep my apps separated between iOS and macOS?
An example of this is the fantastic Fantastical calendaring app. The iOS versions are inexpensive, but if you want the same experience on macOS, it is much more. I think for great apps like this that people put their heart and lives into, we should pay for what we use. If I can run an iOS version of Fantastical on macOS, the desktop revenue stream dries up. Also, any motivation for Flexibits to update the desktop version drops dramatically. For other apps like this, if the developer doesn't make it a universal app, however, I may be less-inclined to run it on all my devices or find an alternative that does.
There are definitely winners and losers in this equation. What do you think?