That seems like a good idea, but in reality most people can't afford something like that. And I would hate to see people not get the benefits of Yoga because they can't afford private instruction.
Personally, I've had a very good experience with classes. I have taken classes from several different instructors and they have all emphasized that there are different options for all poses. They show the different poses/options for each level. They spend time with beginners, and always point out that poses should not hurt. The class participants have always been different levels and it always seems to work out fine for everyone.