For me, I'm learning parts. I could see someone in one style of Karate using this to learn a new style. I could even see someone in mainline NGA using video to learn the core curriculum of Shojin-ryu (if I had videos - not having videos makes video training so much more difficult

). For someone from mainline NGA, the changes to get to Shojin-ryu aren't extreme, so they could make most of the adaptations off some decent video training. And the reverse would be true, too - one of my students could learn most of the adaptations to mainline via video.
Would they get all the way there? No. But if the styles are similar enough, they could make a lot of progress with well-designed video training. Add in some occasional in-person instruction, and the shift can be completed. Again, it's going to take longer than if they have a live instructor the whole time, but it can be done.
I guess I find myself wondering what is the necessity or desire?
Let's say you are already solidly trained in AB style karate. You find some instructional videos of CD style karate, and you find that it is very similar to AB, so you work on it and eventually find an instructor in CD wo is willing to rank you in the new system. So now you have switched over, or at least have added another style to your resume.
Again I find myself with a few questions.
First, since your success in CD was based on your solid training in AB, have you really learned CD, or are you really just still doing AB but they are kind of indistinguishable, with a few minor curriculum differences?
Second, if the two systems are so similar, what is the point in learning something that is virtually identical but goes by a different name? You don't need it, you haven't really added anything to what you do. It becomes padding in a resume.
Third, if you are well enough trained in one system that you can successfully learn another nearly identical system via video training alone, then you don't need to do it. What you already have is solid, and adding something more to it in this fashion is just busy work. It doesn't, in my opinion, add anything to what you were already doing. The very fact that you already have solid enough training that allows you to learn this system via video is the same thing that means you don't have any real need to do so.
Now I agree that ideas and inspirations can come from many sources, including video. But in my opinion, it isn't possible for a beginner to be successful in this manner, and an experienced person who might be successful, based on that prior experience, has no real need to do so.