You guys can shoot me for saying this, but two things:
I think Lexus thought they had it all cleverly figured out by eliminating the GS, creating an F-Sport of the ES and then making the LS "sporty." I'm sure they thought they had their bases covered and the GS was redundant with such a plan. Well, a year and a half into it, we can pretty safely call the 5LS a flop, and IMO, that's due largely because they made a lot of compromises and didn't redesign it like a flagship car. I said originally and still will: The 5LS is more "GS" in execution than LS. It's what the 5GS should have been. If Lexus has any plans of adding another coupe to the lineup, it should have four doors, which brings me to my next point:
It seems to me like the "four door coupe" market is exploding.
Rumored 4 door version of the A9
AMG GT 4 Door Coupe
Coming baby Panamera
Lexus could stand to do something here... you know... maybe ONE model?
People are still buying traditional sedans like the ES and 5 Series, but the models with actual growth are the 4 door coupes. I'm not sure Lexus' strategy of "Lets just give our sedans coupe greenhouses and kill two birds with one stone," has worked. The ES buyer still wants a traditional sedan. I think the LS buyer probably does too. I've been saying it for three years now, but the GS should be an ultra-dramatic, low slung model that's something like a four door LC. The LS should have been more upright and traditional (with better headroom). For a brand like Lexus that still has a fair amount of appeal to rather basic/pedestrian buyers, there's going to be a market for the traditional sedan... that's why ES and LS have done well. They need to stick to that... but also put some effort into this expanding 4 dour coupe market and bring the GS back.
(And no, I'm not stuck on resurrecting the "GS" name. Call it "HS" for all I care - just do it.)