I mean it's difficult to quantify advertising. Most retailers can't depend on just click through rates. If you see a ad for Vans, you might not purchase it online, but like traditional ads, it could make you think about it when you're near a shoe store.
And most normal (non tech/IT people) don't run ad blockers. But at some point there's got to be a low of diminishing return. People are earning less, and in some ways buying more to distract themselves .. yet like any industry, you simply cannot have infinite growth.
What exactly will an advertising bubble bust look like? Or would we not see it directly, as it would be more of an effect of fewer goods being sold due to something else that kills jobs and stifles wages?
How will a dot com break in the 2010s/20s be different from the one in 2001?