Older thread, bumping for my impressions having ridden my first fat-bike. I don't ride fast on trails. I'm a wimp. For one ride I hit frequently though, through two miles of power lines in my community, I could absolutely see this being my only bike. The fat bike gives me more confidence downhill at speed. That path is a convenient short cut that I avoided regularly until now.
I'd probably rephrase this question because one bike will never suit all your needs ideally. I'd ask, "If you had to spend good money on only one bike, would it be a fat bike?" In that case, absolutely. Relatively efficient roadies, for brewery cruises and such, can be had for cheap. The fat-bike does everything else well and you tend to get more as you spend more, relative to a road/gravel/hybrid.
Besides, if you ride for fitness and not distance or speed, the fat bike just gives you a better work-out. Ride it enough and your fitness will catch up.
I love fat bikes.