6000 series cannot run on the Joban Line? That's news to me...^^ as an everyday user (and at times, translator) of the system, I believe after the Hibiya Line's 05 series, the next would probably what I call the "Eidan 1970s Group" which are the 6000, 7000 and 8000 series found on the Chiyoda, Yurakucho, Fukutoshin, and Hanzomon lines.
The 8000s have been updated with LED lighting and LCD screens so they're likely good for another 10-15 years maybe. They can also interline across Tokyu's Den-en-toshi and Tobu's Skytree lines so that's a plus.
The oldest of the 7000 series have been retired to Jakarta whilst the newest have been running as 8 car trains in the Fukutoshin Line or 10 cars on the Yurakucho. However the 10000 series along with Tokyu, Seibu and Tobu trains have taken the urgency to rebuild away.
The Chiyoda Line's 6000 series is a bit stymied since they cannot run on the Joban Line (but are equipped with Odakyu equipment so at times are seen there.) Since all other sets can interline Odakyu-Chiyoda-JR, there might be a reason to upgrade or phase out the 6000 series soon.