Shops stock a lot of expensive bikes now, but we all know their margins are in service. The average rider can do a fair amount of maintenance in their garage with a few youtube videos and a can-do attitude, but where are they gonna go when their motor is on the fritz? I see a new revenue stream for shops.
And niche of a niche, how many of those do we already have? XC, down country, enduro, trail, downhill, slopestyle, hardtail, aggressive hardtail, full suspension, singlespeed, bikepacking, endurance, fat bikes, plus bikes, wide trail, 29er, 27.5, do they still make 26", mullet, and on and on.
You don't see many cross country skis around here either. There are still plenty of fat bikes on the market, and I bet they sell just fine in regions where it snows enough to justify owning one. You are right that you heard a lot more about them when they were new. The new has worn off so they aren't on the hype train as much, but fat bikes absolutely paved the way for boost hubs and plus wheels. Trends tend to overshoot, then regress to the mean. I think that's what will happen with eMTB, and the SL line from Specialized is a prime example. Smaller motor, lighter weight, looks a lot more like a regular bike, just a little cheat mode.