This doesn't make sense, because dollars are fungible. If WM reaps a greater monetary value from the highway system than it spends on the highway system via taxes, WM comes out ahead.

No, because they could be getting even more value by spending the same money that they now spend on taxes, but have that money spent specifically for their benefit, rather than have it be thrown at whatever's politically popular. Yes, they get below cost road usage today, but road costs (due to government management) are also higher.

So it could be that they pay $0.70 to get government to spend $1.00 for 1 unit of road usage, but without government involved in roads, they could buy that same unit of road usage for $0.60. It could go either way.

(Glad to hear we're in agreement on the sense in which the IHS as such is a subsidy.)

I was attempting to convey the idea that that Wal-mart's current (but quite likely ephemeral) success is due to political accidents moreso than "economies of scale". The only "economy of scale" operating at Wal-mart is logistics and trucking, which doesn't scale very much: the planning scales somewhat, the trucking has already scaled as far as it can, and the trucking is on more precarious footing than it looks.

But the alternative(s?) to trucking are even more scale-dependent. What if they shipped goods by rail? That's more dependent on finding huge loads to ship at once. Air? Same thing.