I have seen some subwoofer cabinets designed to face the woofer down though. It's an interesting design, and makes the woofer cone "invisible." But the disadvantage is that the cone weight will tend to shift it downwards, which will only get worse over time.
Speaking of subwoofer facing up or down, I've seen a few subwoofer setups in a vehicle where the subwoofer was facing up. As you mentioned that this type of setup will get worse over time, but are there subwoofers that are made to face upwards?