I will play devil’s advocate
I assume you are only talking about 29” bikes.
100 to 120mm downcountry bikes look awesome, but aren’t they more of a niche for riders who prioritize speed? These may be the bikes that most average riders actually need, but I doubt they will be the bikes many of them choose.
120 to 140mm trail bikes are really the ideal bike “for the average trail rider who rides a variety of different trails.” These bikes, like the new Ibis Ripley, have become great at almost everything. They climb efficiently, rip undulating trails, and crush most technical downhills. The average rider you reference may even be faster on a Stumpjumper than they would be on a comparably spec’d Epic Evo.
140 to 170mm enduro bikes are also pretty amazing. The Ibis Ripmo has arguably been the hottest bike over the past few years and it climbs so efficiently that there is no longer much of a penalty for being overbiked. The average rider in Colorado certainly doesn’t need a Ripmo but there sure are a lot of them on the trails.
So the devil asks – have trail bikes become so good that cross country bikes are almost obsolete?