"The cost of renewables will certainly go down as scale increases"
I think you have it somewhat backwards: the ordinary expectation would be increased per-unit costs with increased scale; but renewables are special because they are a hot technology field, so we are likely to see per-unit cost reductions as technology improves.
The reason that per-unit costs typically rise with scale (as illustrated with an upward-sloping supply curve) is because inputs become more scarce. For instance, the best wind and hydro locations are already taken, so new locations will be slightly worse, leading to higher production costs.
"Economies of scale" are an exception early in the supply curve where it slopes downward (due to better utilization of capital, etc.). We are probably past that point because the scale is already quite high, so the supply curve for a given product using a given process is probably upward-sloping, and there are no more economies of scale to be had.
The higher scales will finance technology improvements (in processes and products) so I agree with your overall point that prices will drop. But the reason for the price drop is technology (the R&D for which may be financed by the higher scale) not the scale itself.
I believe this distinction is important because it means we need to watch the R&D budgets, not the scale. R&D budgets will be flush when firms see a lot of improvements to be had, and will dry up when they don't see much more (even if scale remains high).