Lower costs only translate into lower prices if sufficient competition is there. That is not true for many markets

That’s the big difference in China. When there is competition for everything —> prices are low. Not a lot of profits for investors, though…

Which markets do you have in mind?

I'm all in favour of lowering barriers to entry, too. We need more competition.

Be that from startups, from foreign companies (like from China), or from companies in other sectors branching out (eg Walmart letting you open bank accounts).

Untrue, most of the time. Even with a monopoly, there's still a demand curve.

Would you rather sell one widget for $1000 or 1000 widgets for $10? Does the answer depend on costs?

The ROI for a large corporation tends to be around 10%.