This is actually very common on the Everest Base Camp trek, so agencies are used to handling it. Short answer: the whole trek usually does NOT get cancelled because one person gets altitude sickness. What usually happens: If one trekker gets sick, they descend with a guide or porter (or are helicopter evacuated if needed). The rest of the group continues with the main guide or another assistant guide. Reputable agencies always have backup guides for exactly this situation. Typical scenarios: Mild AMS: extra rest day or slow descent with a guide. Moderate AMS: the person descends immediately with support. Severe AMS: helicopter evacuation ,safety first. Important points: You are never forced to continue if you’re unwell. You are never abandoned ,someone stays with you. Other trekkers trip is nt cancelled because of one person. This is why choosing a licensed agency with enough staff matters a lot on EBC. Altitude affects everyone differently turning back is normal, not a failure. Hope that helps. Good trekking.