The collar's floor is designed precisely for this scenario. If your stock falls below the floor at maturity, you can simply deliver all your pledged shares to settle the loan with no additional obligation, even if the stock has lost substantial value.
Alternatively, you can repay the loan in cash and receive your shares back, though you'd be doing so at a loss on paper.
The key difference from margin loans: there are no margin calls during the term. You won't be forced to sell at an unfavorable time or scramble to post additional collateral.