I disagree. If a transmission is build as how you describe, then it's even more imperative to flush it religiously--because only cycling fluid through can ever hope to hit 99%+ of all of the fluid. It's similar to how you normally never have to worry about the age of the gasoline in your tank, because you refill with new gas prior to draining it completely. Which means that the old gas blends with the new and you mitigate the risk of old gas gelling/congealing (which it tends to do within about 60 days if untreated). Flush, run it & cycle the fluid around, flush, repeat. By sticking to a shorter change interval (eg. 15,000 vs mfg recommended 30,000) you are likely to hit these so called "cannot drain" areas.