I agree, varying the sensation through technique and location on the foot makes a big difference in reducing desensitization. I think a lot of people make the mistake of treating the feet as a spot, as opposed to a collection of many spots. Some people will even laugh differently when tickled on different parts of the foot. Focusing on one foot at a time and moving to the other foot each time desensitization starts can be a good strategy for some. Probably the biggest factor is the individual. The level of ticklishness can vary so much between individuals and how common the different levels of ticklishness are is just guesswork without data. And some people, regardless of how ticklish they are, desensitize very quickly, while some people seem to stay sensitive even under repitition. Can a lee be broken with just foot tickling? Absolutely, under the right conditions and with the right lee. For some, you don't need to worry about desensitization. They'll break in minutes or even seconds! Regarding produced videos, and I can't speak for every one out there, but many are trying to balance between a challenging and expressive tickling experience, and getting a reasonable length video where the lee is not legitimately upset afterward.