The way I interpret the decline of software is that part of the problem is because more software has been written with the mentality that it doesn't need to last 30 years.

It's a self-perpetuating issue: people build stuff saying "it won't/can't last 30 years" for various reasons (money, time, skill, resources, expectations, company culture, business landscape etc). So then software doesn't last 30 years for those same various reasons.

The idea that we think systems did used to last longer is probably some survivor bias. However, software that has survived decades was probably created with a completely different set of methodologies, resources and incentives to modern software.