Let's just take one of your points as an example.

Without those ships added to the fleet the Battle of the Atlantic would have been lost.

If we did not have those ships how do you know that something else would have not come up? While I am sure that it is true that the Flower class did exactly as you say how do you know that, had that class not have been built, something even more effective would have been? You don't. I don't. No one does.

DtG