You're right, everyone does have different definitions, which is fine. There are plenty of schools that, while they may not want to settle for a 5 seed being a high-water mark (and I'm sure they don't), at the same time won't be hanging crepe because they were seeded among the top 20 schools in a given year, rather than the top 16.
In the Big Ten, sign me up for a title any/every year, shared or not.
Yes, what they're experiencing right now is a sharp decline from back-to-back final fours.
The ceiling in this era? Good question that's yet to be answered. I believe a lot of schools are feeling that out. Personally I feel the current transfer rules, where 'commitment' seems to be an obsolete term, are just as big a factor.