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.
And they had 86% adherence for a year. If they'd studied this farther out and it went the same way as basically any study that's done that, adherence would be a lot worse in another couple years. And that's in a group that's exceptionally motivated vs. the general population—has an actual illness they're trying to treat, and has opted in to the study knowing what it's asking of them and for how long.
Pull out method is a less viable method of preventing pregnancy, and especially less viable at preventing STDs. However, it's still considered a contraceptive strategy.
For every 100 people who use pull out method perfectly, within a year, 4 will get pregnant. Since not everyone does a great job doing it, in reality 22 get pregnant every year.
For a going-steady couple that isn't worried about STDs, keeps an emergency contraceptive ('morning after pill') ready for oopsie mistakes, and live in a country that grants women bodily autonomy (or have discussed and accepted that a mistake might lead to a child), pullout is a perfectly valid contraceptive mechanism. There are better, but considering that basically every method except a condom/internal condom can cause extreme discomfort or bad side effects to the woman in the relationship, I think it's totally fine to choose pull-out.
[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.