the principle needs to be relevant for long to be useful well into the 20th century

the phrase beginning with "as its principles" justifies the nature of the impact the law had, by stating that Napoleon's code of law "strongly influenced legal codes well into the 20th century."

The highlighted portions clearly establish that the impact was long-term. The word is enduring.

Why not A ?

but his innovative code of law had a subtler but more _____ impact, as its principles strongly influenced legal codes well into the 20th century.

The highlighted part tells why it had a particular kind of impact. It describes the blank. Since it states that the kind of impact the law had influenced legal codes well into the 20th century, it clearly is talking about the long-term impact of the law. Especially the phrase "well into" highlights the enduring aspect of the impact.

There is no clue in the sentence to suggest the impact was renowned. Something can be enduring without being renowned.