Returns to true scale

If you think you have a case of either increasing or decreasing returns to scale, check again your list of inputs. In the true long run, there are only constant returns to scale.

What you really have when you apparently have either increasing or decreasing returns to scale is an instance of not taking into account that some input(s) can be "fixed" in the sense that they are collective for some range of output production, but this in advance is an instance of short- not long-run production, and therefore, not liable to be analyzed through the returns to scale perspective, which is only applicable to long-run, i.e. all-inputs-varying, cases.

This means that you can never justify that in the long run there are natural monopolies due to increasing returns to scale. What you may have in this sort of cases, it is a firm operating in the short-run first stage of production, where there can be increasing marginal returns on some inputs while keeping fixed others.

See also this post of mine on the same topic.