Thigh strap are not one of "40 add-ons" to a packraft that just make the boat heavier. They are a single addition that we in AK have seen on dozens of boats that give people control and confidence in whitewater from Class III to Class V, as well as leverage for paddling the flat and windy.
People are not getting entrapped -- yet. If a single chambered boat with extra velcro (added to keep a boat dry) were to go flat in the wrong place at the wrong time, then it does seem like being trapped in thigh straps will be bad. Indeed, besides practicing your Eskimo roll in a thigh strap equipped boat, think about letting all the air out while you are in the boat in the pool to see how you'll get out! Simply having a single chambered boat go flat under you, even without thigh straps, is a dangerous entrapment situation and one reason I find the Feathercraft Baylee so attractive. Unfortunately the Baylee is not as maneuverable or boofable as an Alpacka.
It would be nice if Alpacka made two chambered boats as durable and long and lean as the Witchcraft, and if Feathercraft made nimble boats with spraydecks that stayed up. Until then, we'll all be modifying our own boats and paddling them like this: