You can get street view images from maps.google.com for free, so a scraper should be able to use the same interface for free (as long as it's either not throttled, or you can get multiple IPs or don't care about speed).
As you say though, another way to go about this would be to scrape the public SV site, which I have not attempted to do. @xnx linked this project in another comment, which seems to do just that: https://tllabs.io/google-street-view-hyperlapse-youtube-vide...
I don't know if/how they evade captchas.
