It already does that for access points that require logging in (or accepting a ToS). It shows an exclamation point on the wifi icon, and continues to use mobile

It already does that for access points that require logging in (or accepting a ToS). It shows an exclamation point on the wifi icon, and continues to use mobile data. This behavior was introduced to my Nexus 5 beginning with Android 5.0. It's actually a bit annoying—it broke an app I had purchased that could record and replay scripts to automatically log me in to access points, and the app maker can't (last I checked) find an API that will allow them to re-create the functionality.