I wouldn't call it a downfall. It's just a choice. VX Mk1 4:3 was the standard for a long time. It never went away as a standard, but it got supplanted in the main by the Panasonics in 16:9 HD with an Xtreme.
Now people shoot on absolutely any and everything, and make a conscious choice about aspect ratio which is not entirely dictated by the abilities of their hardware. For example, people will shoot on cameras with 16:9 or 3:2 sensors, and crop to 4:3.
I think of fisheye filming in 16:9 as a mistake perhaps partly because of the era I started in, when everything was 4:3. If you started when almost everything was already 16:9, you might have a different perspective on it.
Longlens, I could go either way. Longlens 16:9 is maybe preferable to 4:3, but with fisheye footy the opposite is true because of the issue of cutting off important parts of the image circle, specifically, where the head goes. Switching between 16:9 for long and 4:3 for fish could be done, but would probably be rather distracting. I can't remember if I've seen this done before.