An average "investor doesn't care" about the inner distribution. The question is rather, how does reducing the mining reward reduce the aggregate mining cost passed on to the market ? That's the aim of the DCG proposal. (Although it's stated in terms of miners supplying less Dash to the market but actually, to be consistent with their premise that miners sell to cover costs, it should be restated as "drawing less fiat from the market").

Mining costs are denominated in fiat (because electricity companies). Therefore its the fiat cost that's passed to the market to pay. To achieve the objective of "drawing less fiat" therefore the mining costs would have to be less. For the mining costs to reduce, competition for the next block would have to also reduce. That only happens when demand for the coin overall reduces, not necessarily by changing the reward ratio. (As quizzie has most helpfully pointed out above).

In my observations, a reduction in reward ratio DOES NOT manifest in reduced aggregate mining costs. Instead it manifests in reduced marketcap share and less capital influx.

Since the market (miners or investors) covers the mining cost but only receives partial supply, it reacts by devaluing the balance of the supply it doesn't receive.

When I say "devalue" I don't necessarily mean devalue in absolute terms but relatively to competing mined chains. That's why ranking IS important. Not per se, but because it shows up the opportunity cost of our protocol decisions such as reward splits.