We have a hard time with Shabbos because we don't know how to stop. We must acknowledge the true source of all our sustenance. It's true, that G-d has decreed that we must be involved and responsible for our own destiny, but we must also know when to stop and how to stop. If one violates the Shabbos and continues to work he is making a retroactive statement about his non-reliance on the Holy One. This is because G-d created the world and than said 'enough'.
Creation is dynamic.
A seed is planted, it produces a tree, the tree produces buds, and the buds produce a fruit. It would have been quite natural at the time of creation for the process to continue. The fruit could turn into juice or perhaps a lemon meringue pie. Cotton plants could have continued to develop and grow into material. Jeans and all types of clothing could have been plucked from trees.