'Ignorantia juris non excusat'

This would have been logical with short and understandable laws.

Nowadays, however, it's impossible to know the law in it's entirety. (Nor is it possible to deduce logically from similar known laws).

If it's literally impossible for an expert to know all the laws (let alone a common citizen), 'ignorance of the law excuses no one' is a meaningless sentence used to dismiss a much, MUCH bigger problem.

Try saying it this way:

"Well, you should have known what no human being is able to learn. You're not a super human? Too bad."