In this case the system was deploying new functionality, so your shadow couldn't just run last week's version or you'd get false alarms all the time. And obviously deploying the same broken version to the primary and the shadow wouldn't detect anything.

So you'd need two codebases and two developer teams, coordinated enough that their code produced exactly the same output yet independent enough that they didn't make the same mistakes. With the challenges of coordination this would more than double your costs.

Of course, with the benefit of hindsight, the costs might have been worthwhile...