2000m is not some hard deck, it's not like below it you're good and above it you're *** without preparing for it. About 2 years ago I was in Nepal, at 3500+ for a week. Until that elevation I noticed no change at all, in heart rate or respiration rate, 3000m I was completely fine. It's not like at 1900m you're fine and at 2100m you're struggling, it's much more gradual
it is but the general discussion was that 2000m posed a barrier. There is also the challenge of competing, attacking and responding at an altitude that aren't the same as a measured effort.
Racing repeatedly 2,000 m with sea level preparation; that elevation proved to be monumental in separation from those competitors (Utah, Colorado) that were struggling at lower heights. We actually called the turn up to the Mt Bachelor summit parking lot the "I-5 turnout" at 4,500' because every rider in the front group
not from 6000' could not go any faster or the same speed at the end of an 80 mile race. Those same acclimated riders, used to gradual accelerations at higher elevations were completely shelled in an aggressive criterium at 4,000' the next day.
My guess is their power was a product of the higher elevation pacing and our inability to maintain or adjust pace was equally from not training there. Of course every rider is an individual but a Belgian kermesse rider will not hold the wheel of a 130 lb. Columbian or Ecuadorian climber...likely ever above 2000 to 3000m.