The plan seems to be that the new IE12 will not carry all that cruft, but be a forward-looking modern browser. If you need legacy stuff you must start up another browser.
Internally, when talking to other web devs, you should treat the next Microsoft browser as IE12. Externally, however, when talking to clients and other non-techies, it could make sense to support the “Microsoft is creating a new browser” narrative.
I guess we’ll see the new browser in action at the Windows 10 event on January 21st.