Interesting thread. My thoughts on why people trade in cars to BMW for what they are worth:-
1. It's an expensive mistake - trading a Tesla into a BMW garage is going to cost you dearly. If you were short of cash you would sell privately, so these people are not poor.
2. Poor research - you buy an all electric car without doing the due diligence. It must be a hell of a surprise once you have taken delivery and then discover every problem that is reported in these forums by trial and error. You sometimes see evidence of this, particularly on this forum where, for example, people scream where is the autopilot software and threaten to sue Tesla if they don't get it NOW! OR Why hasn't it got a heated steering wheel, I'm selling the car! In some cases, I am sure they do sell.
3. I'm bored - there are some people in this World whose attention span is short and sell when they are bored of their latest toy. They will claim at dinner parties "I am SO over Tesla!" Avoid these people at all costs as their boredom will extend to you fairly shortly.
4. Patience, Latitude and Forgiveness - most of us do our research and we are mentally prepared for the Tesla ownership experience. We will put up with the trials and tribulations of a new power source and a new car manufacturer, count to 10 (or higher) and move on (with the help of exemplary Tesla after sales support). Most modern cars these days do not go wrong but the Tesla does. You cannot own cutting technology that is updated constantly without accepting that this is part of the deal. Some people simply will not put up with problems but this is probably because they have made the mistake of no/poor research.
Once BMW bring out a car that does not "cheat" with a hybrid/range extender engine, then I am sure we will see more trade ins as a true All Electric BMW will muddy the waters in terms of choice. However, clearly Tesla are a company that enjoy and thrive on taking risks so they will continue to have a substantial lead in technology over all other car manufacturers. I relish this as most of us do and look forward to a future which does not include trading my Model S into a BMW dealer!