How many world records have been broken at high altitudes. I bet you there are none. The fact is, although your body acclimates, it can't produce the same intensity on less oxygen.

If you have a scheduled competition at a higher altitudes then it does make sense to get there a week or so early so that your body can adapt to those conditions. Otherwise there's more benefit in training at a higher intensity at sea level than the lower intensity at altitude.

I heard one idea that's supposed to be a win-win. Suppose the athlete trains at sea level, and sleeps at high altitudes. Then he will be able to have the benefits that come from the acclimation, and the intensity of training at low altitudes. I haven't heard of anyone carrying this idea out, but if it doesn't cause any bad side effects effects then it's worth investigating for those with the means.