If you think Android and iOS are "done", consider where they are headed and you can see they are barely getting started. Consider input such as speed, physical gestures, breathing, brain activity. All manner of form factors. Integration with the "real world" such as Nest and Square. Wearables, cars, home appliances ...
Whatever early-stage web APIs exist for these things are vastly, tragically, fragmented across the various HTML5 runtimes.
The "other web", that of HTTP and REST and JSON and cloud services, is a freaking wonder of engineering and a giant productivity boost to developers of any client. But the UI side of the web is continuing to trail for now. Competition among tablets and mobiles will help native apps as much as it will help the web.