Every conference has bottom-dwellers. You need them to build the top end.
You actually need MORE teams so that your top teams in each division don't really play each other very often during the regular conference season. That way, your conference championship game matches top teams with nice-and-shiny win-loss records and higher rankings. Boom. Follow the SEC model. It works.
The Big Ten was following the SEC model, and it has been working. But this year, they moved back a couple of steps in favor of the PAC 12's nine-game scheduling model and having a lot of marquee matchups in the back half of the schedule. It will ultimately hurt the Big Ten. Sure, the Big Ten has six ranked teams right now. But, they mostly still play each other, which means more guaranteed losses for your top teams. Most of the ranked Big Ten teams still play 2 or 3 other ranked conference opponents. That will eventually squeeze a couple of Big Ten teams out of the rankings.