When I was actively teaching many years ago ('78 - '05 or so) we still ran eight, two-hour sessions in the classroom, followed by 90 minutes of pool time, then a weekend of open water diving. Even at that, I really only touched the surface, but to this day, I feel like we birthed some knowledgeable divers. A bunch of my students are still friends to this day.
PADI came in to Canada about the same time as I started teaching and over the years, they reduced to requirements to get someone certified but replaced that with multiple courses to keep up the training. It seemed like a good plan, especially from the shop's perspective, since you can keep extracting money from the same people, but more importantly, you keep those same people coming back for supervised dives, which should help with keeping them involved. The problem comes when people don't keep coming back, since there is no solid base knowledge. It also became increasingly difficult for what had become "old-school" instructors to offer a competitively-priced SCUBA course. The programmes weren't remotely comparable, but to the couple signing up for a class, they had no idea whet the difference was. Ultimately that led me to retire.
Clearly PADI has come to dominate the world of recreational training, and most other agencies have come to adopt similar training approaches.
I've believed for the long time, that a happy-medium would be a good option... less than 8 weeks, but more than a few hours in front of a TV...