Agreed. Except on the few flights I've taken in the past year that departed with empty seats, bin space seems nearly always to be at a premium and rarely have unutilized space. Granted this is not the result of a detailed study, rather empirical observations. As an extreme example, anyone taken a weekday morning SFO-BOS flight on UA in the past three months? Seating 3.5 and above wind up gate checking bags.

The pilot referenced in the OP's conversation may or may not have been informed, but charging for bags as well as the Travel Options mentioned occasionally in other posts are all part of the "unbundling" business model that gained momentum when fuel prices were sky-high and continues as the legacy carriers necessarily identify new ways to be profitable.