General practice today has a defined number of doctors, taking on hospital tasks in ever-increasing numbers and complexity, looking after sicker patients because of the inability of the hospitals to holistically manage patients, and having to satisfy an ever-increasing - and justifiable - demand from patients for appointments.
The study on patients controlling appointment lengths was carried out on too few patients for too short a time. It does not answer the question as to whether such a system is sustainable in the long term.
Further, such a system may not give the GPs the opportunity to practice opportunistic health checks and health promotion. It may make some patients withhold problems they have because they see that they are running out of time during a consultation.
I cannot see how a surgery with a defined number of doctors can plan its work with patients choosing how long they think they want to be with the doctor.