[dekhn]

As pointed out by others, they were very cost effective. And they replaced vacuum tubes, which were fragile. But, the most important thing is that they immediately solved a problem (ballistic trajectory calculations in missiles) and therefore had an industrial use which led to a virtuous cycle of improvement.

Quantum computers aren't useful; they're still searching for a use.

[vbezhenar]

I think that NSA and similar organizations would be interested in breaking encryption algorithms. Imagine some old archive encrypted by RSA which contains useful data.

Also that means that encrypting anything valuable with quantum-unresistant algorithm and storing it in an unsafe place is not wise even today.

[dekhn]

We can assume that NSA maintains a hardware portfolio and that quantum computers would be a tiny part of that. I'm sure NSA has been able to decrypt RSA for a while. https://www.reuters.com/article/us-usa-security-nsa-rsa/excl... and https://en.wikipedia.org/wiki/Bullrun_(decryption_program) and https://www.pcworld.com/article/2064960/google-strengthens-i...

The history of decryption hardware is fascinating and filled with practical solutions, which QC is not.