I always keep my 4K TV in mind when I record anything, even if I’ll watch the videos more on my iPhone and iPad. Portrait is usually suboptimal in my view with few exceptions like a baby walking (close up).I disagree on this one. My videos are for me and my family, not for you on iTunes or youtube. If I'm taking a video of my baby walking, then a vertical video will get more of him in the frame and thus capture more of the moment. And by the way, the vast majority of views of this video will be family and friends on iPhones and iPads, so the vertical works quite well.
That's just one example and yes, people should consider portrait v. landscape each time they start recording, but this is far from the hard and fast rule so many people would have you believe.