There isn't any obvious reason it would drop over time, I think the bigger question is how will advertising evolve over time.
For a known brand ads serve as a means to say "we are here, we are alive, we do well". That's why Coca-Cola or McDonald's keep running ads, despite the fact that everyone in the world is aware of them. This is not unlike wearing an expensive necktie on public, or having a gold watch 100 years ago; it's the same sort of signaling a peacock uses his tail for.
If you stop putting your ads before public's eyes, sales start to slump. Maybe this is not so for a brand with a small cult following, but such brands are few.
So advertising is not going to go away, as much as luxury items market is not going to go away. It may shrink a bit when times are hard, it may migrate to different media (from paper to TV to computer screen), but not disappear.