How should travellers think about altitude sickness in Nepal?
Altitude is not a fitness test you can win by enthusiasm. Build acclimatisation days into the route, know when to descend and make sure insurance and evacuation assumptions are real before you need them.
Answer
Altitude is one of the few Nepal planning issues that should make even confident travellers boringly conservative. Being fit helps with walking, but it does not make you immune to altitude illness. A good route climbs gradually, protects rest days and leaves room to turn around without destroying the whole trip. Symptoms are not something to push through for the sake of a pass or base camp photo. Insurance should be checked before the trek, especially for altitude limits, rescue and helicopter evacuation assumptions. The mountain will not reward bravado; it rewards people who listen early and descend when the plan stops being sensible.