Google already mapped a large amount of WIFI hotspots by gathering the signal strength and MAC address data during the Google Street Views project. This information is used for example when you enable geolocation feature in your Firefox browser.
Excerpt from https://www.mozilla.org/en-US/firefox/geolocation/
How does it work?
When you visit a location-aware website, Firefox will ask you if you want to share your location. If you consent, Firefox gathers information about nearby wireless access points and your computer’s IP address. Then Firefox sends this information to the default geolocation service provider, Google Location Services, to get an estimate of your location. That location estimate is then shared with the requesting website.
Most of the people with Android phones also backup their WIFI hotspot data to Google. Now Google has map of hotspot locations, combined with who owns these hotspots, so its very easy to tell the precise location where someone with an Android phone lives.
Now imagine only the precision they can achieve with PokemonGo. Not to mention all the other possibilities that the OP talked about.
The spooky thing is that Google already did the mapping once without asking any user consent. I see no reason why they won't do it again.
Samy Kamkar also mentions how Google mapped the WIFI hotspots in his amazing DEFCON talk:
Thanks @deepbits for such an interesting facts.
Yes, Google, Apple and other big companies use power of crowd in order to massively collect data. This approach as also known as crowdsourcing. It's applied to gather info about Wi-Fi/Bluetoth access points that's used then to refine positioning accuracy in their maps. But it hasn't been applied yet to perform visual mapping. and I think PokemonGo is a big step toward this direction!