George Lucas did more for the advancement of motion pictures than anyone since Lumiere brothers.
I think it'd be more accurate -- MUCH more accurate -- to say he did more to change
motion pictures than anybody since the Lumieres.
Digital wouldn't have evolved as quickly, and certainly wouldn't have caught on at the speed it has, if he hadn't spearheaded various processes in sound, VFX, editorial and cinematography ... but since those changes haven't always necessarily improved matters, but did make it possible to work much faster or at a greater volume of output (note I'm NOT grouping digital compositing in with this, because that seems to have been a win-win through and through), and since it has been in service to films that percentagewise seem significantly inferior to their predecessors ...
And no, GL is not singlehandedly responsible for dumbing down the industry. But it isn't coincidental that after JAWS and then GL's big pic, that the supremely impressive era of filmmaking we saw in the early 70s has never recurred. I mean, for every EARTHQUAKE there was a THE LAST DETAIL, for every Bronson pic there were as many Coppolas (for a little while anyway) ...