You are probably correct, and so was I:Schrader valve tubes can be found for every size "except for the ultra thin types only found on road/racing bikes/wheels sold at pricey LBS". Thin Tires/Presta tubes are often found on enthusiasts bikes; just look at the bikes on this list. But I would not call "obscure" every other bike tire/tube size sold and ridden in the U.S., including the vast majority of bikes ridden for every purpose by cyclists of all ages except perhaps for touring, racing and enthusiast/club activities.