I think the point is: Suppose google blackout for a day, since they have never done this before afaik it will give them insight into how crucial their service is (or isn't) to the population in general.

If they saw that modern society could not function without them then they could then effectively hold legislators over a barrel and say "pass law X or we turn it off", these laws might not always be things we would like.

Of course if they pulled that enough times I'm sure another engine would come along and replace them.

This is a problem I have with centralized software and data that is proprietary to one company, I imagine Google and Facebook could legally do some pretty nasty things if they wanted to.