Richard,
As a former superintendent turned agronomist I thrive on plant physiology. You didnt get my point yet supported it at the same time. Of course a trees canopy is there for a reason and serves a purpose. The size of the canopy always equals the size of the root mass, because as you mentioned photosynthesis, enough photosynthesis is required to support the root mass through carbohydrate and protein production. The EFFECT of that canopy CAN provide less competition, but it doesnt ALWAYS work out that way. Most of the tree work done at my last course was eliminating and thinning out smaller trees directly around and under the larger, nicer trees so they could be accentuated.
When I said this....
The adaptaion of nature is the "biological" result and the biology will always evolve.
....that means if a tree is removed or foliage is permanently lost, like you said, the competition will take over. Nature has adapted to the loss of that tree or the growth of it.
The ability of a light source to create a shadow is in itself a physical abilty. All it takes is a light source and an object to block that light. Creating a shadow. Therefore the shadow doesnt exist for the plant. The plant exists, or doesnt exist, because of the occurrance of a shadow. So no, the de is not there as a defense for the tree. The tree is there because its seeking sunlight and at the same time casting a shadow in the sunlight. The biology around the tree adapted to the lack of sunlight and as a indirect result the tree lessened its competition.
...and after overthinking this myself the original question was why does shade still exist after years of evolution and its effect on agronomics.
Shade will always exist. As long as there is a sun and something in the way of the light waves. Agronomically, its a pain in the ass to the super. Architectually, courses do get overgrown and it gets ridiculous. But there are many cases where the architect plants or keeps the tree to serve strategically.