However, about 20-22 mm would be a nice compromise for a high BL hybrid 21" that could be used for pro sound and home theater applications. That is a niche that continues to go unfilled, not that many will champion such an approach. I guess that is why the niche goes unfilled eh?
Doubling Xmax means a 6 dB increase in output (if you have more than four times the power available to achieve it) which is a big deal- at 20 Hz it sounds more than twice as loud.
Going from 15 mm to 20 mm is only about a 1.5 dB increase, but the production cost probably goes up more like 3 dB..