Wow. With all the other topics that are discussed here without a comment from an ASA, it gets your attention when an ASA not only comments on a topic -but actually starts it.
That leads me to "assume" lots more people than expected haven't voluntarily switched to using only the new interface and the AdSense team has noticed.
My question to ASA and the AdSense team, even if it only becomes a rhetorical question, what's the downside of keeping both - the current interface for those who want the simplicity & the efficiency that brings them and the new interface for those who want the features available there?
P.S. On a related note, and FWIW, I no longer use AdWords to advertise my business because the process just became too complex. There's virtue to be found in keeping things sweet and simple, IMO.