Unfortunately, in practice the "simplest implementation that works" is understood as the crappiest implementation that seems to work.
IF you are young and naive, you could argue that this was an honest mistake. In every other case, trying to push to a client the "simplest thing that works" it is either professional negligence or borderline fraudulent cost externalization.
Not being paid by the hour is not a valid excuse. It is a symptom of either naivety or shady business practices.
Trying to push to a client the crappiest implementation that seems to work is certainly everything you say that it is - but that's not the same thing as delivering the simplest implementation that works.
IF you are young and naive, you could argue that this was an honest mistake. In every other case, trying to push to a client the "simplest thing that works" it is either professional negligence or borderline fraudulent cost externalization.
Not being paid by the hour is not a valid excuse. It is a symptom of either naivety or shady business practices.