The volume is in starter bikes. They have massive long-term investments into huge fixed costs (factories) that can only be ammortized through enormous volume.The Industry™
In the end, the big 3-5 will survive and determine that it's only worth selling $8,000-15,000 bikes and drop the "cheap stuff"...
More likely in this scenario is that if 3-5 bigs only survived, they would no longer compete on brand (ie stop selling the $3000-$10,000 bikes), go direct-only (ecomm + brand owned stores), and stop supporting racers/races/etc and only focus on the volume. The expensive stuff is simply loss-leader marketing to sell townies and kids bikes for Giant, Cannondale, Trek, etc.