Something designed from the start for a specific purpose will usually be better than adapting something else to fit. There is normally a bit of leeway with a design and some variation is feasible but there comes a point where a new design would be a better option. The DC8 Super 60s series was stretched to about the longest practical length possible for a narrow body and any further increase in passenger numbers needed to be accommodated through increased width rather than length. Had the A380 been a commercial success there was the possibility of increasing the length but there wasn't enough demand for an upgraded variant.