If you want to argue about this case - use facts that apply to this case.

Are you really arguing that a debt cannot be sold because the new owner didn't contract with the debtor?

Mortgages are sold all the time.  The new owner of the debt is certainly owed the money.  It depends on the original debt instrument.

In this case, the Santander was the serviced the debts for the original lender and ultimately purchased the loans from the original lender. Santander is not a third-part debt-collector, they are a financing company that makes loans and services loans.

These loans were not assigned to Santander to collect debts - Santander was collecting payments on a portfolio of loans and forwarding the payments to the lenders.  Santander purchased the loans from Citi and continued to collect payments for the loans that it now owned.  I'm struggling to see how this makes them a third-part debt collector vs. the financing company that they are.