Actually a couple of economists back in 1952 did a survey of factory managers and found that only about 5% associated rising marginal costs with rising output at all levels of production. The remaining 95% faced either constant or decreasing marginal costs. Here is a reference…

Eiteman, W.J. And Guthrie, G.E., (1952). ‘The shape of the average cost curve’, American Economic Review
42: 832-838.

In this sense software is fairly typical, though extreme in that its marginal costs are particularly low as you say.

“only about 5% associated rising marginal costs with rising output at all levels of production”

…should read…

“only about 5% associated rising marginal costs with rising output at any level of production”

The marginal cost of distributing software is not 0 (at least not for most commercial software companies). You have to include the costs of selling, and supporting the software. The cost of sales can be quite high – look at the annual reports of publicly listed software companies – the Selling General & Administrative expenses are 30 – 50% of revenue!

I believe that if you account for these expenses – this wil result in a supply curve that is not very different from the classic ’supply curve’.

If you only consider linux (or other freeware) – the supply curve is clearly a flat line – the suppliers will supply all the software you need for 0 – they have no selling expenses, and do not support the product. That does not apply to most commercial software providers.

How one analyses things depends on how one chooses to partition and
represent reality and so definitions of terms are clearly important.
My analysis so far focused on two bits of the total process of the
production and use of software: the production and distribution
aspects alone. I have neglected the next bits which include the two
bits you point out — sales and support. I will add those two bits later.

A picture is worth a 1000 words. For non-economists like me, a diagram which shows the curves you are talking about would have been very helpful !

I bet that one can do better. I bet that one could even construct an economic theory that would reasonably describe software — its manufacture, distribution and use. In fact it’s been done.

Economics is a broad way of thinking that goes beyond the specific material in Econ 101.