A cottage window replacement is a construction job: scaffolding on a slope, a crew on site for days, and a schedule that has to fit a road that may not take a delivery truck in April. What most owners want from that spend is narrower. The room should stop cooking at four o'clock, the floors should stop bleaching, and the great room should not be on display to every boat in the bay.
Each of those is a surface problem, and a surface can be treated without opening the building. The honest limit: none of this fixes a failed seal, a fogged unit or rot in the frame. If the glass itself has gone, we say so on the first visit.