You could then walk away with a 10-15% gain on your money on current BTC prices - no one would do this. If you increased your collateral to say 35 - 40 BTC people would be more interested.
This is true. If you are making a fiat based loan then you would probably need to give escrow at least 120% (but some may want 150%) of the loan value as collateral in the event that the price of bitcoin falls during the loan.
I am not sure why people try to get a fiat based loan secured by bitcoin. I would think it would make more sense to me for the borrower to simply sell the bitcoin on an exchange as this is what they are doing anyway plus it would save them in interest costs.