Perhaps that's because most people who think that the claims are BS have never worked with codebases that are expected to exist and be maintained for at least a decade with minimal downtime.

Within large companies those types of codebases are normal. The upfront development cost of applications is significantly less than the long term maintenance cost - especially once you've factored in the cost of hardware over the time period, and having to keep institutional knowledge of the applications amongst development teams, sys admin teams and network teams.