I'll leave the "academics talking about Star Wars" to others who probably know a lot more about this subject than me, and I'll throw in some non-academic stuff. I haven't studied this in detail (so my analysis is shallow), but I am an American so I feel I can give opinions about stuff without being burdened by the need for any research or facts.
To me, the most interesting "cultural landmark moment" was when the term "Star Wars" was used politically to discredit the US SDI program in the 1980's (to portray it as unrealistic/infeasible/"science fiction" as opposed to an achievable technology). I think this is interesting because it's completely outside the typical film/literary context and also because the term was politically very successful (indicating it resonated with the US population). Counterarguments are that this happened in the heyday of the Star Wars phenomenon (concurrent with ROTJ) and doesn't indicate a lasting influence, and that SDI failed for far more serious and concrete reasons than a simple political slogan.
You can get some basic quotes and info here:
As far as I know, cultural influence can only be measured by inferring from the number of references over time. i.e. William Shakespeare has more cultural influence than Christopher Marlowe in the 21st century, but they were probably equally influential in the 16th century, and you could measure/prove/disprove this by how many other authors cribbed lines and concept from their writings over time. So you've probably seen all of these lists below, but I'd be remiss not to include them because IMO this is the solid measurable stuff. Of course, you may need to compare this against other American cultural landmarks to see how it stacks up: I'd suggest Casablanca or Wizard of Oz for other iconic films and the Beatles (yes, I know they're British) for a non-film comparison. Certainly academics may have already looked at this and made their conclusions, but I find these lists interesting in their own right.