Run a study of your approach and show good results over a cohort for 2+ years and you'll make an entire sub-field of medical scientists very happy.
[EDIT] It's not that your approach can't work, it's that if (for example) people had as hard a time following the directions for condoms as they do following diet & workout plans, we'd never allow condoms to be sold as contraceptives, they'd not even be close to being OK to promote as useful for that purpose. It wouldn't matter if a few people could follow the directions and it worked 100% of the time for them.
[EDIT] It's not that your approach can't work, it's that if (for example) people had as hard a time following the directions for condoms as they do following diet & workout plans, we'd never allow condoms to be sold as contraceptives, they'd not even be close to being OK to promote as useful for that purpose. It wouldn't matter if a few people could follow the directions and it worked 100% of the time for them.