I’ve worked on some agile projects from others that I picked up and had to "fix", where we, now, after 2 years of trying to work with them, have decided to completely rewrite them from scratch.
Every agile project I’ve seen yet was just a huge ball of technical debt. It worked, it had few user-facing bugs, but adding features was impossible, and the documentation was "whatever the software does". Completely matched the agile manifesto "working software over documentation" and "change over plan".
Your response is selection bias. Every project that you've seen is a ball of technical debt. There is no argument to how documentation alleviates technical debt. If you document a poorly designed and scoped piece of software it doesn't make it any easier to change.
If the software was impossible to change and that change was a requirement then the code bases you were working with did not represent "working software"
The agile manifesto says, "working software over documentation." It does not say "working software and an absence of documentation."
The fact that you needed to add features is exactly the reason agile exists. If the creators had a complete feature plan in the beginning and faithfully executed it without deviation from the plan then there would be no need to ever add features and the quality of the code in terms of "changability" would be moot.
As compared to the waterfall projects I've witnessed, most of which never made it to completion, the rest of which were far away from what the client actually wanted once delivered.
Unfortunately, outside of a small percentage of companies, most software written today is just bad.
