Interesting discussion......
Having owned a fat bike many years ago, I think the trend has pretty much faded away. I rarely rode it and really only seldom miss owning it. Pretty much the only time it was worth owning was when there was a couple inches of snow on the trails and even then I only rode it a few times a season. So for me, over hyped.
The gravel bike on the other hand, I freaking love. It's not road bike fast on the road but it works well enough to get you to a rail trail, canal etc in a timely manner and is so much more comfortable on those surfaces than a road bike would be. Yes, you could ride a drop bar xc but I don't have the time to be swapping out parts like that multiple times a year. So, if I didn't own one, I'd either just wind up riding my XC MTB or just not ride those paths at all (more likely). Which would be a shame because as
@goldsbar mentioned the roads are not that safe.