Were battlecruisers a failed type?

Would the losses prove that battlecruisers were a failure?

Not inherently. Battlecruisers become dangerously exposed when used against threats their protection was not meant to withstand, while their intended roles remain valid.

Battlecruisers are not inherently failures. They work when used for the roles for which their speed and protection were designed, but become dangerously exposed when employed in tasks better suited to battleships.