I don't really think that the problem is lack of media budget or lack of money spent updating the GS and the IS. The real problem is that buyers clearly prefer the more established and prestigious nameplates of the 3/5 series and the C/E class, so in a shrinking luxury sedan market, these established players are going to squeeze everyone else out (just like how the Camry, the most established medium sized sedan nameplate, is squeezing out all its competitors in the market, including the arguably superior Honda Accord). The only way Lexus can compete in sedans now is by having a product which doesn't compete directly with the Germans (hence the ES).Again Lexus succeeded with the FF because 2 main reasons,
1) Toyota give Lexus this chance , by the poor quality of handling , fit & finish in Camry and Avalon until this current generation.
2) Lexus itself by not giving the GS & IS media promotion with 1/4 the media budget they gave to the ES
We all want to believe that a stylish new 5GS would reverse the fortunes of the nameplate and bring sales back. I think realistically though, a new GS on GA-N, while competent, would not be enough to overpower the 5 series and the E class.
I support Lexus' decision to focus its attention on crossover projects like the LF1, where hopefully it can build its brand prestige.