But the question asked why you would bother, not why someone who has been married for 40 years would bother. You don't need to know anything about anyone else to answer the question.
It is recognized that you did answer the question. You said as a defence mechanism. That is understood. But the follow up is asking on deeper level: What needs to be defended against, exactly? What is that you think your ego going to do to you if you don't put up these defences?
The last coherent thought you had was an assertion that YAGNI may include not implementing extension points. But then DHH, while admittedly starting with a made up premise that made no sense, came along and obliterated that notion by the end, detailing how extension points – what he calls model, integration, and system – are necessary to keep tests sane.
And, since YANGI emphasizes testing, we can be assured it actually does include implementing integration points.
It is recognized that you did answer the question. You said as a defence mechanism. That is understood. But the follow up is asking on deeper level: What needs to be defended against, exactly? What is that you think your ego going to do to you if you don't put up these defences?