[toohotatopic]

That happens if everything goes wrong but you cannot create that situation intentionally. Usually, pushing the emergency button would send an emergency signal, and air traffic control would clear the airport. Landing on top of another plane is an emergency within the emergency - very, very unlikely.

[foldr]

You can't just "clear" a big airport. There's also no way to guarantee that the emergency signal would be received.

[toohotatopic]

You also can't guarantee that you won't be hit by a meteorite and yet you leave your house.

If you worry about the signal not being received and the inability of air traffic control to spot the irregularity, then only initiate safety landing when the plane receives an answer-signal. Otherwise, let the plane fly however it wants and let it crash somewhere randomly because most places are better equipped to handle a random plane landing than a regular airport.

[foldr]

I think you're forgetting that the button itself would only be useful in extremely unlikely contingencies. But sure, you can spin a story about a bunch of interconnected automated systems always working correctly in highly unusual circumstances. Doesn't usually pan out that way in practice.