"If you have trusted services available, you may not need a blockchain" assuming you can actually trust those services. In the U.S., among people in lower economic strata, this is not actually the case.
Matt Taibbi's recent book The Divide has a chapter on the abuses of our credit collections system. People are getting taken to court of debts they never actually owed in the first place, or already paid off. The lender has sold the loan to someone else, the records are lost, the borrower get "served" at an old address and never actually hears about the case, then loses to a default judgement for not showing up at court...and finds out about it when wages are garnished.
Here's a simple idea I came up with to fix that:
http://www.blunderingcode.com/ethereum-credit-cards/
It still uses the courts to enforce payment, but it uses the public blockchain to stop this nonsense about lost records and fraudulent attestations that they exist, while still protecting borrower privacy.