Interesting thread.
I'm not sure that the cost of flying is the determining factor in drop-out, though it may hasten the day for many.
It's been said already that you "have to fly for a purpose". I think that this is a significant factor in drop-out.
An analogy. We each have a car; and motoring is very cheap. How many of us take our cars out just for a drive around? Not many of us, I'll wager. I wouldn't dream of getting into an aeroplane just to stooge around the local country-side.
I'm pretty confident that the piloting drop-out rate is comparable to other activities. Once the individual has achieved his goals...PPL, going solo, or whatever...., it is quite reasonable for him to move on. Back-packer's views most closely echo my own.
It's probably true that most PPls are ill-equipped for cross-country work, but that isn't the schools' fault. There is every opportunity for more adventurous flying than that which is prescribed in the PPL' syllabus...if you are prepared to pay for it.
As to hours'-building instructors...well, you don't have to fly with them. Fly with the instructor that you want; don't be fobbed off; you are the customer, so tell the school that you want to fly with an experienced instructor.