It's like nukes. No-ones actually dumb enough to use it.
To make money, you need customers. If isp's use dpi, they have no customers.
And while the average joe doesn't know that an isp is using dpi, they might not be the person administering the account.
Gods knows I set up my parents internet. My parents contribution was reading the credit card details to me over the phone.
And most of my friends ask for my feedback before switching isp's. At least the happy ones do