Comment by lelandbatey

Yes, new major airports (rare as they are) do try to acquire large areas of land, larger even than they think they need now, in anticipation of future expansion. However, for scenarios like this, there's limited utility to making the runway longer "just in case." They already pick runway sizes "big enough and then some" as the minimum to even bring planes of each size to an airport. So there is margin.

But no matter the margin, a plane can always crash on the wrong side of any fence. And people will always build right up to wherever you put the fence as closer to the airport is more convenient for everything airport related.