Let's be honest - all the ES really needs is an active AWD system that can split the power delivery between the front and rear wheels for consumers who want either more power or better driving dynamics... and the case for building a GS becomes approximately zero. This is what Audi has done for years with every sedan in their lineup: you can buy the plebeian FWD model if you don't care and just want an appliance, or you have the option of AWD, sports packages, larger engines, etc. It works for them and it can work for Lexus, and this method would represent a major risk aversion for Lexus when sedan sales are tanking dramatically month by month.
Judging from the spy pics, this is a good looking car... much more svelte and interesting than any ES before. If they offer 300+hp ES F-Sport that looks this good, they probably will be able to keep ~50% or more of GS buyers, all of their current ES audience and eliminate significant cost and risk.
If there is a small displacement turbo V6 put into an ES F Sport with active AWD and it produces, say, 350hp, very few people will care about the death of the GS.