Why would you train a pure neijia? What is the disadvantage of learning something that is not fully neijia? Are the qualities of kuzushi on contact, technique arising naturally from a dantien-powered movement, non-telegraphed strikes, and powerful strikes with no wind-up characteristics of a neijia arts? If so, does this suggest that those qualities are watered down in systems that are not considered "neijia" in your eyes? If not, what is it about neijia that sets it apart from other arts that are not neijia? What besides those martial qualities I described above does neijia offer that pure non-neijia arts don't? How do you known that what Sam, Ark, or Dan does not offer what pure neijia offers and how do you judge this? What is about a neijia that makes you want to focus on it and not on the distractions?
Why would you train pure neijia? Because they are fascinating arts that I've been reading about for a long time, and the qualities you mention are attractive. I don't think no-wind up strikes are 'watered' down in non-neijia arts, they are just more powered by local muscle. One advantage of the neijia is that you can deliver significant power without having lots of muscle. But such strikes, done externally, certainly can work (and they are certainly easier to learn and use more quickly, two not insignificant advantages to the waijia). But if they are powered by local muscle (upper body muscle) rather than Dantien-controlled whole body power, all the time (check the requirements for taiji, bagua, xingyi movement), then they by definition aren't neijia. And one thing neijia masters say over and over is that if you want to get good at those arts, you have to go all in, re-training your body to move from the Dantien is hard, even harder if you're training it some of the time and training external arts some of the time.
Ark was actually quite disdainful of Dantien-powered movement when I saw him last (even though, as I mentioned, it was clear he sometimes used such movement). But it's not 'all Dantien all the time' with him, and again, that is one key definition of the neijia.
What makes me want to focus on the neijia? What makes me love Tim O'Brien novels? Just crazy like that, I guess. I'm an amateur, this is a hobby for me...they're fun and difficult in that way that all engrossing hobbies should be. If I were solely interested in defending myself or winning fights I'd still be doing muay thai and kali escrima.