Strictly speaking, rational choice theory does not claim that people are "complete" human beings in a metaphysical sense. It is a formal model that simply assumes actors have preferences and make choices under constraints. However, when that model expands beyond economics and becomes a general way of understanding society, it can begin to carry exactly the implication you're describing.

The rational-choice actor is typically presented as already possessing preferences. The theory generally does not ask where those preferences come from, how they were formed, whether they are still developing, whether they are internally conflicted, or whether they emerge through relationships with others. Preferences enter the model largely as givens.

As a result, the human being appears less as a creature in formation and more as a chooser navigating an environment.

The central question becomes:

Given what you want, how will you choose?

rather than:

How did you become the kind of person who wants these things?

or

What kind of person are you becoming through these choices?

This is precisely one reason thinkers like Alasdair MacIntyre criticized modern liberal and economic theories. MacIntyre argued that they tend to begin with an abstract individual who already has preferences rather than with a person whose identity is formed through traditions, practices, relationships, and moral development.

Similarly, Charles Taylor argued that modern thought often treats people as "self-defining choosers" while neglecting the social and historical processes through which selves are constituted.

Your second observation—that choice architecture itself appears complete—is also important.

In much rational-choice analysis, the world is modeled as a set of constraints, incentives, opportunities, and costs. The actor encounters this structure and responds to it. The structure may change, but it is often treated as externally given from the individual's perspective.

Critics such as Karl Polanyi and Michel Foucault objected that this framing can obscure the fact that social arrangements are themselves historically produced. Markets, property rights, labor relations, educational systems, and legal frameworks are not natural facts like gravity. They are collectively created institutions.

Yet when viewed through a rational-choice lens, these institutions can appear as fixed features of the landscape within which individuals must optimize.

This is where your point about consequences becomes especially significant.

If individuals are assumed to be competent choosers navigating an already-given choice architecture, then failure tends to be interpreted as a failure of choice. Poverty becomes a result of bad incentives or bad decisions. Unemployment becomes a problem of insufficient adaptation. Debt becomes a consequence of poor optimization. The focus naturally shifts toward the individual's navigation of constraints rather than toward questioning the constraints themselves.

This is one reason some historians trace a line from Townsend's idea of hunger as a labor discipline mechanism to later incentive-centered thinking. If hardship is viewed primarily as information—a signal telling individuals to adjust their behavior—then suffering acquires a disciplinary role. The pain of deprivation becomes part of the system's feedback mechanism.

Critics argue that this can lead to a subtle moral transformation. Instead of asking:

Why is someone hungry?

the question becomes:

What incentives does hunger create?

Instead of asking:

Why does this social arrangement exist?

the question becomes:

How should rational actors respond to it?

From this perspective, the individual is not understood primarily as being formed by the world or co-creating the world. The individual is understood as navigating the world.

Many alternative traditions reject this picture. Aristotelian ethics, pragmatism, phenomenology, psychoanalysis, Marxism, feminism, communitarianism, and various religious traditions tend to view human beings as unfinished, relational, and continually formed through participation in social life. In those traditions, both the person and the social order are always in the process of becoming. Preferences are not fixed inputs; they are among the things that must be explained.

So I think your formulation captures a genuine tension. Rational choice theory as a formal tool does not explicitly claim that people are complete or that social arrangements are complete. But when its assumptions become a broader social philosophy, it often encourages a picture of already-formed individuals navigating an already-given incentive landscape. Much of the criticism directed at rational-choice thinking comes from traditions that insist both the individual and the landscape are unfinished products of ongoing historical and collective creation.