Acoustic aircraft detection as a dead-end technology

What is an example of a military dead-end technology?

Acoustic air-defence listening systems: they made sense before radar, but radar made them obsolete as a primary solution.

Acoustic air-defence listening systems absorbed resources without delivering a durable result. Their large horns and arrays became an evolutionary dead end once radar offered a much better means of detection. The original investment was still rational within its period. Before radar, an approaching aircraft's noise was one of the few available signals.