How many people want to take that much time off work, or spend their holiday, learning something that will put them in the same position as those who don't. They'll wind up at the same vacation spot, diving with the same fellow tourists, being led by the same DM. How many want to forgo those holidays to save the money needed for that same course. How do you sell them on the idea that it is something they need.
Have them spend a couple of hours reading the "Near Misses" subforum here. Read about running out of gas, losing buoyancy control, losing buddies, Read the threads elsewhere from people who want to know how to keep their video from being shaky, or how to take a better photo. Better skills mean safer diving, less stressful diving, and more fun, as you can spend more of your attention on the things you actually want to do.
90% of my diving is single tank, open water diving at recreational depths, often with a camera. Doing the GUE training meant that, when Peter and I got swept away from the group in strong current in the PI, we stayed together, shot a bag, and surfaced where the boats could still see us. GUE training meant that, the night Peter thought he'd gotten bad gas, we established an air share and got safely and comfortably back to shore, after a very clear discussion of whether we needed to ascend right where we were, or could swim closer to shore. GUE training is why my dive buddy and I today spent over 60 minutes on a beautiful dive site, moving in and out of clefts in the rock by going forward into them and backing carefully out. It's why I could swim over to my buddy at his signal and descend to about an inch off the bottom, to peer under a rock at a beautiful big octopus, and then ascend and back up without disturbing anything at all. It's why I could hang, absolutely immobile, for about ten minutes, among the stalks of the bull kelp, watching black rockfish, shiner perch, sand lances, Puget Sound rockfish and pile perch swirl around me.
Better skills means better diving. Better teamwork means less stress and more fun.
I think the GUE technical classes are excellent, but there IS excellent technical training available elsewhere. Where I firmly, passionately believe the GUE approach to diving has the most to offer, is to the simple recreational diver who never intends to do anything else.