I would agree if this was about "bad" code. But maintainability is easy to prove -- just give some random developers a feature or two to implement and see the results.

In every single project I ever joined there were people complaining about the quality and maintainability of the code. Yet some of the projects were truly stuck not able to deliver anything and others were chugging features at a steady pace.

Maintainability is difficult to define but it is definitely real.