I hear that the WT cabin on BA is rarely more than 75% full except on some very popular routes.

If this is the case (and I might be wrong?) then why not add a bit extra legroom or a but more width to the configuration and maybe charge a little more for it. An extra 3" in WT would mean a loss of about 10% of the seats, thus the only loss will be excess capacity which is rarely used and with the potential upside of more pax and more revenue due to the differentiated product.

This would make me happier as I am 6'4" and find all airlines WT a pain (literally). In comparison to this cramped environment, just the extra room of WT+ feels like a dream and Club is even better !