The evidence speaks for itself. About 10 (or 15?) years ago, American Airlines introduced "more room in coach"; removed some rows in Y and gave everyone in Y more leg room. I used them and it was significantly more comfortable than BA, etc.
However, this did increase their costs, and they didn't get an real improvements in their load factors, so after a couple of years they went back to "normal". So the evidence is clear; many Y passengers are driven primarily by the price of the ticket, and airlines are pretty much all minimising costs while maximising revenue.
That's today's airline business; if the 787 does reduce costs it will not lead to lower prices per se but to increased profits / reduced losses. And the airlines will (somehow) all continue to charge pretty much the same prices, unless you are prepared to do some fancy routings, where you trade your time against the price of the ticket...