It's true that the only way for proper rust remediation is replacement metal, blasting, or EvapoRust for smaller stuff depending on how advanced the rust is. I don't use POR-15 because, as noted above, it doesn't stick to clean metal, it is very brittle making it susceptible to cracking from flex and temperature variations. I have used Eastwood Rust Encapsulator for decades and have never had a failure. Mitigate the rust the best you can, spray two coats of rust encapsulator followed by a coat of 2K primer and your topcoat. It sticks equally well to clean and rusted metal. I only use this for areas that would be very difficult to blast, or weld-in replacement metal. But to put this in perspective, if you are working on a $5K car, you don't necessarily make the same choices you would on a $100K car. Many of our cars have had rough lives. Most of us tend to be a little more careful that blasting down salted roads, so it's harder to gauge the effectiveness of a coated piece. The rust shown on the '74 is typical surface rust found even on California cars, and if it was garaged in a low humidity locale and not driven in weather it would likely look the same in 10 years with no treatment of any kind.
A real world example of severe abuse is my pool heater. My wife put a bag of fertilizer on top of it and over time the caustic material leaked out and destroyed the finish. When I removed the bag, I had about a 10"x10" area that was rusted and deeply pitted. I don't like honey-do housework, so I figured I'd wire brush the area hit it with Rust Encapsulator. I didn't even prime it, just top coated with some Rustoleum I had lying around just to get it overwith. That was about 5-6 years ago so I went out to take a look. No sign of any finish failure or rust of any kind. It sits in direct sunlight and exposed to rain. It's reached the end of it's life cycle and being replaced shortly with a heater that uses some poly-plastic derivative because these thing are prone to rust, but it certainly did hold up well. Actually the internals gave up the ghost far before the top was ever an issue again. If you decide to give it a try there are 2 different products now. The old cheaper formulation is the one to get. The new and improved premium version is more like POR. You open the can and whatever you don't use is waste. It will lock the top of the can closed no matter how spotless it is when you put the cover back on. The old stuff is like primer and can thinned with lacquer thinner to spray. It can be sealed back up without a problem. The can I used on the pool heater was actually 10 years old. Took a heck of a lot of mixing, but still did the job.