True for devices under your control. But think venues with large BYOD counts. Add in that all client devices generally transmit at full power. End result is an environment with not a lot of headroom in the 2.4 space.
2.4ghz isn't the only problem - even cell towers have problems with large amounts of devices. And "large BYOD" events are not a normal use case for wireless, and even 5GHz will have problems in those situations.