I think that it is important also to note that many of the records hidden behind Ancestry's and others' paywalls, especially the ones that they obtained early in the development these sites were sourced from records contained in the FamilySearch collection.
Most of these records were not available online at FamilySearch in the past. However, the recent change in FS's goals involving digitizing their entire collection by 2020 means that many of the films that Ancestry (and others) used are now viewable online at FS with a couple of caveats:
1. The images may be online but have not yet been indexed so there is no search function on them yet.
2. The images may be restricted in viewing to either a local FamilySearch library of an "affiliate" library. Since there are FS libraries in almost every LDS chapel in North America and I assume in many others around the world, this means viewing them is not any great ordeal for most of us.
3. There are a few, very few, records that are restricted to the main FS library in Salt Lake City. For most of us these are pretty-well unavailable unless you are rich enough to hire a professional genealogist.
The trick when looking at a source in Ancestry is to see if there is a FH film number or call-number. If there is, then the film exists in the FS collection. The next step is to go to FS and search the catalog for the film number. The entry found will show you whether the film has been digitized, whether the images are searchable and whether they are restricted or not.
I have been using this method to find free sources for Ancestry records for both my own family and as a Data Doctor and Unknowns Project member. One needs to remember that the FS search engine basically sucks. They have a lot more information online that their rudimentary search will locate. You often need to really get creative when digging for information you know is there, you just don't know where.
Another trick when running up against book sources behind paywalls, etc is to check both Google books and Archive.org. Many of the older genealogical books are online at either site and more are being added every day.