Were the K-class fleet submarines a sound idea?

Were the K-class submarines a good concept undermined only by poor technology?

No. Their technology was badly executed, but the underlying fleet-submarine concept was also deeply flawed.

High surface speed was meant to let submarines maneuver with the battle fleet, but achieving it produced complex machinery, awkward handling, and dangerous transitions between surface and submerged operation. Better technology alone would not have made the design sound because the fleet-submarine mission imposed contradictory requirements.