[lmm]

There's oversupply if more inventory is available than can be used effectively. If you accept the idea that there could be an oversupply of e.g. lumber then it doesn't seem too much of a stretch to say the same could be true of advertising space.

[bduerst]

This is survivorship bias.

What you don't see are the spots that have disappeared because there wasn't enough demand to be profitable. The examples you've given are being used effectively because the price points have dropped to match demand for those specific service spots.

Advertising follows user attention, and as attention shifts mediums so does the advertising. That doesn't mean there is an oversupply in the advertising industry, it just means that it has liquidity (more so than physical goods like lumber).