Street View is hopelessly out of date in many places (4 years in some parts of Sydney for example) and it has to be super expensive to keep thousands of people continuously driving around the world. And Street View will likely be a vital and profitable data source if VR/AR (in all its forms) is ever used en masse.
Sounds like too much expense for just that. If the problem of street view was it's cost you could just pay people to add a StreetView system to the roof of their cars and get the data much cheaper than having your own car. They may very well want to start by using Street View to validate the technology but the end-game must be larger.
I think you're underestimating the number of people you'd have to pay to get full coverage of cities. Most people do not drive around on every street in a systematic way; they have a small number of destinations and they usually take the same routes between them every time.
That doesn't mean that once you do have self-driving cars you wouldn't use them for that. Or even that StreetView wouldn't be the first thing you'd use them for, since that's a good way to get more training data for your algorithms. But to suggest they'd go to all the expense of creating self-driving cars to then use them just as a cost saving measure for StreetView sounds strange.
Not unlike the experience for drivers on Lyft and UberX it would be trivial for Google to create a rig and an app. You set the magnetic camera rig on your roof and sign in. It shows you available routes and how much they'll pay for them. The offer increases every so often if nobody selects the route. Done.