The optimal tub volume to wash a load is significantly less than the minimum volume to properly dry the same load with blown-air tumble drying. It's possible that without a leap to a different drying technology the dual-purpose laundry appliances will never work well.
In the UK I believe domestic washer-dryers outnumber standalone washing machines with standalone tumble dryers. I assume this results from smaller property sizes than the US. If you live in a city in a newer property - there is no room for two appliances rather than one.
Isn't less better when hot air drying? In my experience when I've had very small loads due to accidental spills they dry quickly. I do understand that more water dilutes detergent, though.
Yes. That's the point. The volumes I was referring to were the volumes of the appliances' tubs, not the volume of the load. The assumption is that the load is constant since it needs to go from washer to dryer.