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).
But it can’t be approached like e^−x either, because the marginal cost of hardware starts to rise once you get low on resources.
Edit:
Exponential decay looks like this
Whereas he marginal cost curve looks like this
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.