Interesting question! You are correct that the earth is not expanding or contracting, therefore new crust formed at mid ocean ridges must be balanced by recycling of other crust back in to the mantle. This process occurs at subduction zones. The Pacific Ocean is probably the best example of this. The East Pacific Rise (the mid ocean ridge in the pacific) spreads at a very fast rate. The oceanic crust that is created at the ridge spreads outward and is eventually recycling back into the mantle in subduction zones, for example, below the Andes, Cascades, Aleutians, Japan, and New Zealand. Volcanoes that form above subduction zones make up the Ring of Fire around the Pacific Ocean.

The Himalaya and Mt. Everest is a slightly different setting. This is an area of continent collision. There was once a subduction zone in this area, but it was basically “clogged up” when India collided with Asia. India is made up of continental crust, which is much less dense than the underlying mantle. Therefore it cannot easily be subducted into the mantle; it’s like trying to get an ice cube to sink in a glass of water. India and Asia continue to converge, but neither can sink into the mantle, and that is why large mountains form in such areas. The uplift of the mountains is balanced by erosion. In fact, the Himalaya are probably near a maximum height that mountain ranges can reach. The faster that mountains uplift, the faster they erode.

Mount Everest is actually still growing about 3-5mm per year! But perhaps eventually the plate tectonics in the area will alter and it will not continue to grow. Tectonic plates shift around due to slow magma currents under the Earth's surface. It is because of these currents that the surface of the Earth is constantly changing.

That is a very good question. The reason that mountains do not keep growing up indefinitely is because of erosion. Although the mountain keeps being uplifted and growing higher, the peaks are eroded. This erosion takes places in different ways. Two common examples are erosion from the freeze/thaw cycle and erosion from rain. The freeze/thaw cycle is that water gets into the cracks between rocks, freezes, and expands. This happens over and over and over and over...breaking off chunks of the rocks over time. Due to gravity these rocks will eventually fall. It might be hard to imagine that erosion can keep the mountains from growing up but both tectonic uplift and erosion happen over thousands and millions of years. When tectonic uplift rates exceed erosion rates, the mountains grow higher (e.g. Himalayan mountains), when erosion rates exceed tectonic uplift rates the mountains grow smaller (e.g. Appalachian mountains).

In one word: erosion.

The higher mountains get relative to surrounding terrain, the faster they are eroded. This prevents mountains from getting too high, because eventually erosion catches up with rate of uplift and the mountains stop growing.

Additionally, mountains that get high enough tend to break up along fault lines that result from the limited strength of rock. I believe it is currently thought that the Himalaya are close to this limit already, but that doesn't explain why all mountain ranges aren't as high as the Himalaya. The real reason, as I said, is erosion.

The collision of plates is indeed ongoing and may create mountain ranges, but it exists in an equilibrium with the effects of gravity and erosion. Thus the Himalayas (and the Santa Ynez Range) are both being simultaneously uplifted and worn down.

Erosion takes place through the action of wind, water & freeze thaw, (and in the Himalayas, water in the form of glaciers), breaking rocks into smaller component parts that then wash or roll down slope, answering to gravity. This is why old mountain ranges that are no longer being uplifted, erode to lower & lower elevations over time.

Additionally, the Himalayas are interesting in that it has been suggested that they are also collapsing under their own weight, sloughing off marginal units much as a tall wedding cake might press down on its center and slowly push the lower layers out laterally.