Looking at the original post...
I have spent some time studying the history of photography and there seems to be some relevant material here that might offer some context to the "fill the frame" principle.
The earliest cameras had several challenges including (but not limited to), poor optics and heavy, unwieldy equipment. This often meant that the photographer could not fill the frame on many occasions because of terrain, distance to a subject etc. The advantage that they did have was large format cameras, so they could crop (and they did) to get a reasonably tight image. This was in the age before Kodak came along with the first cameras that democratized photography - an endeavour hitherto limited to those with the education, time and financial resources to take photographic images with complex gear and often dangerous chemistry.
Cropping by serious and professional photographers has continued throughout by renowned photographers such as Man Ray and Lee Miller. Ray often shot wide and cropped as part of his creative process, and in their collaboration Ray and Miller created some stunning images from crops. Today such revered photographers as Elliott Erwitt have cropped heavily - the image of the small dog with legs was massively cropped.
Kodak's cameras brought photography to the masses, initially by allowing a rising middle class to purchase and shoot images at a relatively modest cost and with no formal training required. Still, the negatives were small and optics were not great, so getting the biggest image of the subject in the frame was critical. It was here that the principle of "fill the frame" became established. It was a simple mantra for those without technical knowledge or education.
The "purity" of the un-cropped image was enhanced by groups such as the f64 Group who wanted images so carefully composed that they needed no cropping - so suddenly the principle was elevated from a pragmatic advisory to the unskilled using scratch gear, to a mantra for high art imagery.
Photographic technology has moved on a LONG way since then but some of the same principles hold true in context.
1. The Kodak camera has been replaced by the cell phone with obviously better image capability, but for those using them without attachments, the principle of "fill the frame" is still quite important - the sensors are tiny, and are often wide angle, so cropping significantly is not desirable.
2. There is a wide range of subjects out there and some will never be photographed "up close" - the astronomical comments I have read are a case in point. As commented also filling the frame with an apex predator is not wise unless under very controlled conditions.
3. As the range and sophistication of cameras and lenses has improved massively there are extremely high-resolution image cameras on the market that make cropping to get an otherwise impossible image a viable proposition.
So is "fill the frame" still as relevant as it once was? I would say Yes and no. Desirable if one can, and yet having space to crop for effect or to change the subject balance is now more viable than ever, allowing for more decisions to be made post production. And there is still that need to get the otherwise impossible shot.