Seat occupancy a day or two ahead of departure is not a good indicator of the demand for premium seats. US carriers upgrade their elite customers in the days leading up to travel. For example, AA offer their ExPlats (akin to BA Gold) a number of complimentary upgrades.
Many's the time I've seen J cabins on US/AA ex-MAN go from very quiet a week out from departure to much busier on the day. Part of the problem is that the frequencies and timings ex-MAN are not attractive to business travellers. That's one of the reasons why a lot of business pax from our neck of the woods still trek down to LHR to pick up better timed flights.