The glideslope (as in the transmitter that is part of the ILS) is aligned to the aiming point and usually 3 degrees (can be steeper). If you follow that down to the ground (eg for an autoland) you won't hit the keys. On the approach plate it will state the TCH (Threshold Crossing Height), which as below for YSSY 34L, is 52 feet. This is a relative height, so actual height for 34L is 66 feet.
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But if you were descending on a 3 degrees decent profile to the keys on a visual approach, you would still not hit anything as the clearance starts from the end of the strip initially at 2 degrees and later at 2.5 degrees, as per the following diagram.
Usually if an aircraft lands well short, the first thing to go will be some of the HAIL globes (High Intensity Approach Lighting - aka the Christmas Tree) - but you've got bigger problems if that happens.