Is the pool of 4 down towable vehicles diminishing??

Definitely diminishing. Honda and Toyota have NO flat towables I'm pretty sure even the sticks. Same for VW, Mazda. Chevy has several models for now as does Ford. But you have to be careful and actually consult the owner's manual for different models and equipment. You can't go by the towing guides. Almost all of the towables have 1 model you can't tow; eg,, Sonic RS, Fiesta ST, etc. Some but not all of the problem is the spread of CVT's of which I think none are towable.