There are far fewer independent bookstores than there were 20 years ago, largely because chain bookstores prospered and multiplied during that time.
Evidence
There are far fewer independent ↑CB -cause→ ↓IB bookstores than there were 20 years ago, largely because chain bookstores prospered and multiplied during that time.
The shortage of independent ↓IB -cause→ ↓VB bookstores has prevented the variety of readily available books from growing as much as it otherwise would have.
Conclusion
Chain bookstores’ success has ↓CB -cause→ ↓BC been to the detriment of book consumers.
Evaluate
The argument assumes that a ↓VB -cause→ ↓BC reduction in the variety of books available to consumers is to their [book consumers] detriment.