People often go into PhD, like I did, to do research and contribute to the public good. Most people like me dreamed of an academic job with tenure. Tenure so that you can actually do radical work without retribution.
That environment no longer exists. Tenure is highly fought for and there are very few positions. And even if you get tenure, you really don't have the luxury to work on anything you wish because of funding. A good portion of your days as an academic consist of teaching, another good portion is writing grants. If you don't do the popular thing, you don't get grants. If you don't get grants you don't get students.
And sadly, without students that means that you don't get to do much research because the majority of your time is teaching and dealing with grants and other bureaucracy. As my former supervisor put it: you get time to do research yourself when you get home after work.
So you have time, but not much. Most profs use this time, but it's just not enough without grants and students.
Ok, so let's go to industry. You get paid a lot more. You get to do primarily research. But it's not public. It's mostly locked up and directed to ultimately lame purposes like targeting ads or making trades. These help no one and don't help society, so it's really not very enticing to an academic hopeful.
And yes, these researchers do fall off the radar because they stop publishing. They may file patents, but the only people who read those are lawyers. This is because patents are not papers. Their primary purpose is law, and they don't contain any of the information an academic would be interested in. You generally can't reproduce a work from a patent, and patents don't include any requirements of proving your invention works, which is the primary requirement of science.
To make matters worse, the patent office typically patents extremely simple things that aren't even worth the attention of someone trying to achieve state of the art. Essentially patents are a huge joke, so nobody looks at them, either in academia nor industry engineers.
TLDR: people go into academia to make the world a better place and do novel research that gets published to the world for free. Academia is increasingly a bad way to achieve this and industry doesn't achieve this at all. Though industry does get you paid so ends up being the fall back, even if you fall off the map.