If we walk away with no deal now, we will not be subject to the backstop (unless we later choose to sign up for a backstop which seems unlikely).In what way is walking away with no deal now better than accepting the withdrawal agreement which lets us carry on more or less as if we were still a member while negotiating our future relationship? It seems to me the only risk is that we have to walk away with no deal later.
If we accept the withdrawal agreement then the backstop becomes law by binding international treaty, and if the EU wish they can then keep us in it indefinitely.