The company just announced that beginning in July, 2018, administrators will be required to give all users the ability to opt into the new interface. Then, sometime in September 2018, all users will be switched to the new interface by default, although the option to switch back to the old interface will be available for approximately one month. After that, the option to use the old interface will vanish, and all G-Suite users will only be able to use G-Mail using the new interface.
The company has not made any official announcement regarding users who have free G-Mail accounts. However, most industry insiders expect that given the timetable outlined above for G-Suite users, free G-Mail users can expect an email or other communication from Google about when the option to use the old interface will be going away for good. Ultimately, Google means for everyone to use their new interface design, and will certainly enforce that.
Google's handling of the change has been exceptional. Change comes to us all, and in business, sometimes it can descend at a terrifying pace. Only by slowing things down to a more human scale can you give your employees time to adapt and grow accustomed to the coming change.
Kudos to Google for a job well done, and business owners, take note. Change may be inevitable, but it doesn't have to be scary. Just give your employees time to get used to the idea.