That’s a marginal cost curve at a fixed time. Its shape is not directly relevant to the long-run behavior; what’s relevant is how the curve moves over time. If any fixed quantity becomes cheaper and cheaper over time, approaching (but never reaching) zero as time goes on, then the price goes to zero in the limit.
Consider Moore’s law, for example: the marginal cost curve for compute looks U-shaped at any particular time, but over time the cost of compute falls like e−kt, with k around ln(2)/(18 months).
That’s a marginal cost curve at a fixed time. Its shape is not directly relevant to the long-run behavior; what’s relevant is how the curve moves over time. If any fixed quantity becomes cheaper and cheaper over time, approaching (but never reaching) zero as time goes on, then the price goes to zero in the limit.
Consider Moore’s law, for example: the marginal cost curve for compute looks U-shaped at any particular time, but over time the cost of compute falls like e−kt, with k around ln(2)/(18 months).
Until you hit a hard limit, like lack of resources.