Many of the arguments against using a ramp from the other question is valid for this as well. Any ramp means a height difference between the ramp itself and the runway, for it to actually make a big difference I suspect a fairly long slope from the ramp to the runway is needed, and this will obstruct the runway for landing aircraft. If the ramp is above ground, aircraft taxiing for take-off would also have to overcome the upward slope, meaning that more fuel is needed for taxi.
You'd also need a ramp at the opposite end; the runway would basically look like a stretched out 'U'. A possibly unintended consequence of this is that the aircraft would at some point reach the point where the runway begins to slope upwards again, resulting in the aircraft either having to abort or use additional power to get airborne.
To avoid such a case you would have to stretch out the runway so this isn't an issue, and if the goal is to reduce the runway length the current design is more efficient. Alternatively you could create a separate runway for each direction, but then you'd be faced with potential crashes whenever a pilot tries to land in the wrong direction. This would also result in difficulties for airports located in built-up areas where there simply isn't available room for additional airport infrastructure.
If you keep the ramps on ground level and the runway center below you'd also need working drainage pumps whenever it rains or snows. If not, the runway would quickly be reduced to a pond with very wide roads leading into it. Depending on where in the world you are, water might seep in from the surrounding terrain as well, making reliable drainage even more important. This is generally avoided by building runways with a slightly raised centerline so that water runs out to the sides so that you can use "passive" drainage systems such as pipes, ditches, pools etc where no machinery is required.
There are some airports such as Lukla where one end of the runway is substantially higher than the other, but due to surrounding terrain it is only possible to land and take-off from opposite directions. Landing aircraft can use the upward slope for an additional braking effect, while aircraft taking off will get a speed boost from the downward slope.