>Then maybe it's time to quit punditry; that was 8 years ago and the online advertising market has grown 2.5x since.

Isn't the very nature of bubbles to get inflated? 2.5x or more?

The main difference compared to a solid venture is not that they see growth (that's what they do best!), or for how many years they see growth, but that it's an inflated (hence "bubble") growth that can (and eventually will) disappear overnight.

A non-bubble industry will also see growth, and will also see decline. But it will not see a huge sudden drop from hero to zero, unless something extraordinary happens (e.g. a war, alien invasion, etc).