It's not fading away. It just peaked. There was an explosion of fat bike sales from ~2014-2021. Now the majority of people that wanted a fat bike have one so sales have slowed.
With niche use for most as a winter or beach bike and the settling of hub size and 1x drivetrain there probably will not be another explosion of fat bike sales again. I have a 2018 carbon fat bike with 1x12 drivetrain that can fit 4.8 wide tires. There is nothing offered in 2022 that would be any better and pursuade me to upgrade.
Rocky Mountain tried to shake it up a little by offering a slacker head angle Fat Bike (which will be worse on the snow) and Otso just dropped a steel fat bike that can fit 5.05" tires but good luck finding may tires that size. They won't be big sellers by any means.