Well, I wasn't around in the 70s, but I guess I'd be curious what those "alternatives" were.
Recently I watched a reality TV show. They selected a few pairs of identical twins. Each pair was put on a diet of identical calorie count, plus identical exercise regimens. One of the pair ate 100% plant-based, the other ate a conventional mixed diet.
Long story short, the plant-based diet has distinctive advantages in overall health, mood, alertness, etc.
On the other hand, a diet doesn't have to be entirely plant-based to achieve such benefits. I always say we're all vegans to an extent. If you have a meal with meatloaf, green peas, and mashed potatoes, you're basically two thirds vegan in that moment. I think people get turned off by diet concepts where they have to be 100% one thing or another.