I would certainly agree that where there has been serious organisational failing, and especially where there is no clear individual culpability by any member of that organisation, the head should resign / be sacked - they take the big pay cheque, the buck stops with them unless it can be clearly delivered elsewhere. (I wouldn't agree that it should have been Cressida Dick who went though - the organisatonal issues were not hers and if you actually read the detailed sequence of events she was in a no-win situation because of deployment decisions taken by others. Ian Blair was the one who should have resigned - it was the whole organisation that failed and several of those failings can be traced to strategies overseen by him.Sharon Shoesmith (personal opinons of her aside) was not directly responsible for the death of baby Peter, however, she was responsible for managing the department, and the staff in that department, who failed to protect him. she was dismissed; Cressida Dick got promoted.
People say lessons are learned, but they're not.
Not often enough they aren't ... partially because of this constant distraction in looking for scapegoats - we need to investigate organisational failings in cases like this in the same way that we investigate air accidents (i.e. to ensure that no more planes fall out of the sky by finding what happened and why as quickly as possible ... even if that means the accounts of those involved cannot, by law, be used as evidence against them ...)