What is this reasoning?

If you know the right way and wrong way to build a bridge, you'd build it wrong, because it makes no difference?

More like, if you know English and Latin, why not blog in Latin, since it makes no difference to your ability to enjoy your posts.

So, the question that makes the difference should whether he writes blog for himself or for his readers?

[replace "blog" by "code"]

Anyone can read their blog! Since anyone can learn Latin, if they feel like it.

(Yes, I know I'm representing a terrible argument.)

It's a lack of reasoning, if it's not causing any issue's why change? The rest of us write good code because we're to dumb to read bad code.

No, the reasoning is why build it to handle 600% specified load (and higher cost) if 150% specified load is sufficient?

(Not that there aren't good reasons to do this. Especially for public infrastructure which might be around for a shockingly large time. But predicting future usage is very difficult. For a small start-up, it's fair to err on the side of leaner/fix-it later IMO)

The other way around. If you can get out with doing it the wrong way, why even care about there being a good way ?

And here you see why one should be wary of hires who are too close to the computer, e.g. most CS graduates.