Citigroup CEO Jane Fraser highlighted some potential benefits of lower interest rates this week.REUTERS/Erin ScottWall Street's biggest banks face a less profitable lending environment as interest rates fall.Bank bosses shrugged off those concerns this week and pointed to several benefits of lower rates.They cited stronger demand for loans including mortgages and refinancing, and improved repayment.Lending threatens to become far less lucrative for Wall Street's biggest banks as interest rates fall.