It's supposed to be the busiest. Otemachi is also being redeveloped these years.
More of a Sobu corridor problem. Will be alleviated by Sobu-Keiyo through line and Keiyo Line common section quadrupling, with some relief further inwards from Yurakucho Line Tokyo Line 8-11 branch extension.
For some historical background:
Before Narita Shinkansen failed, there was a Keiyo-Yurakucho line through service concept floating around.
Toei Shinjuku Line was devised to extend towards Hokuso Line had Chiba New Town didn't collapse, although how much this would divert Musashino and Noda lines may be doubious.
Noda Line had a Tozai Line interrunning branch studied.