I took "identical" as in feature wise. The user should be able to accomplish the same things, no matter what platform they're using.

I do think that the way the features are implemented should be platform dependent, i.e. use common UX pattern on each platform, fit in with the UI, and be good platform citizens.

The solution is to make the common features a part of a statically-linked library that you can pull into your apps. A web app is, IMO, never the answer when you want to address multiple audiences. It is bad for accessibility (because 99.999 percent of web app developers never consider accessibility) and it doesn't solve the problem that web apps are trying to solve (having 8 code bases all that have to remain in sync) because you will inevitably need your code to handle platform-specific differences, standards/conventions and whatnot.

Why does Shopify need to use Apple's or Google's branded design language in their app?