It's not just small businesses. As discussed in a post today ( https://news.ycombinator.com/item?id=26385317 ) Google suffers from this too: they've got advertising revenues down to less than 90%, but this is still very risky. They have a "tech" premium; normally Wall Street investors are uncomfortable with lopsided revenue exposure.
The thing is that if you want to reach people you need to go where people is.
If the air comes out of the on-line advertising business Google faces an existential challenge (as did the newspapers due to the development of online advertising). They have scraped together less than 15% of revenue from a motley collection of marginal businesses (e.g. Nest) while lack the internal management signals to seriously engage anyone in improving things. For example, I assume that if Dianne Greene left Google Cloud it would simply wither. She has the experience and fortitude to actually make a serious go of it. I seriously doubt there's anyone else in the company who would -- it generally doesn't encourage such people.
An imbalance in revenue has an additional problem for investors, which is blinders. Rockefeller and the other owners of the Standard Oil Trust made significantly more money after the antitrust breakup than they had as a monopoly.
Radio went from premium and premiere programming to disk-jockeys and news/talk.
Trying to recall source(s), though possibly Ithiel de Sola Pool.