the fact that a system eventually becomes complex and flawed is not due to engineering failures - it is inherent in the nature of changing systems

it is not. It's just that there will be some point, where you need significant effort to keep the systems structure up to the new demands {1}. I find the debt-metaphor is quite apt [2]: In your scenario the debt accumulates until it's easier to start fresh. But you can also manage your debt and keep going indefinitily. But in contrast to financial debt, paying of technical debt is much less obvious. First of all it is pretty much impossible to put any kind of exact number on it. On the other hand, it's very hard to tell what you actually should do to pay it off. (tangent: This is why experienced engineers are worth so much: (among other things) they have seen how debt evolves over time, and may see the early signs).

[1] https://tidyfirst.substack.com/p/the-openclosedopen-principle

[2] https://blog.pragmaticengineer.com/tech-debt/