I voted 16 because why not (practicality aside). But there will always be a +1 issue here, I think. Teams just left out. This year has a strong top 8, but there will be years where a lesser team would get #8 seed consideration, leaving other teams, such as the ones that beat them, wondering "why not us?"
Especially with how fluid the playoff rankings are, which is a good thing.
The issue with guaranteed conference champions is when a conference has a down year and don't produce a playoff-worthy team. It could certainly happen in an 8-team field, which leaves only three at-large spots. Look at the NFL with its 12-team field and the state of the NFC South this year. People are calling for a change there, too.
I think the most fun would be 16 teams with guarantees for *all* conferences. The Sun Belt can try for a Cinderella story, but they'll have to go through the top-seeded teams to do it. And having guaranteed champs in this case gives nearly free byes for the top seeded teams; they just have to take care of business against likely a nobody, rather than a Top 20 team. So teams will really want to earn a #1-4 seed during the regular season, just like now.
As for the watered down regular season, it's already happening/happened and while it sucks for nostalgia's sake, I don't think anybody will really care in the end. Even with this 4-team playoff, I think we see conferences doing all they can to protect their playoff hopes. The SEC does the best job with their scheduling practices. I don't think the incentives are lining up for quality regular seasons anyway.