Might it be that shemitta is about what belongs to God and is given to us as a source of abundance, and that shemitta does not apply to the fruit of our own labors, which is the product of scarcity?
When God describes Himself as the landlord and the people of Israel as the tenants who have a claim only on the produce of the land but not on the real assets, it could be He is referring to the matter of how to allocate the over-abundance that can come of a too-well-ordered society. In that case the laws of shemitta would correspond to the Biblical admonition not to strut with too much self-importance at the acquisition of cities already built and fields already plowed when the Children of Israel occupy the land of the Canaanites, etc.
The Chumash is dealing with the problem of how to allocate in the face of abundance rather than scarcity, where markets can allocate effectively and, likely, efficiently. The answer to the question of how to distribute rents is: the sabbatical – to fund an annual 15% excess every seven years, and then once every 50 years to spend a year when the only thing happening is happening on the inalienable side of the economic equation.
The inalienability of the land at Yovel is reflected in the inalienability of the produce and of the debts and of the sale of persons into slavery. Shemitta is a mass de-commodification of the economic order. The super-abundance that comes of having too much rent is distributed like a reverse head tax, a sort of head subsidy, one per person, to fund the work of the society that would be done even if there were no compensation, out of the motivation of the people's creative impulse and self-expression rather than in response to the incentives that come from trade and compensation.