>what incentive does a for profit company have to shift any gains from economies of scale/etc to consumers?

You'd have to have listen in to a Walmart upper management meeting to see why, but my guess is that all commerce is somewhat fungible. There's always a bit more growth to be eked by lowering prices or increasing value of products.

I'm not saying that this always happens, or that it isn't shared with increasing profits. Gigantism in box stores could be used in areas for monopoly pricing where they have wiped out smaller competitors (who themselves wiped out smaller competitors) but there are numerous cases where it hasn't happened. I'd say that fear of anti-trust action is only part of the reason.