The ability to unlock an Identity with a pin number, I do not think would be any different to the present day system. How easy is it to get past a pin number?
I do not have an alternative solution. Also , Just like now, Once you use your number, The company you use your number in/with, how secure is that.

Where as the blockchain cannot be altered, Once a number is used it is out there. I think and again I do not know this solution. A system where we can be sure the person using an Identity we can be sure the user is that person.