Surely if 100% of the public wants something they can articulate _some_ justification?

Inevitably: "we've always done it exactly this way until now, and it's worked, and we'd rather just keep doing it exactly this way, without having to re-think it from scratch."

Where "this way" includes all sorts of implementation details that may or may not be important.

Think of a baker in some kind of baker's guild, who was taught as an apprentice various techniques for how to get the best rise from their bread, before any scientific understanding of what those techniques are doing. The baker wants to keep following their various rules of thumb, because they know they'll produce the best bread. They don't know why the rules-of-thumb work; they just know that whenever they've accidentally diverged from them, they get less puffy bread.

Now, replace "baker's rules for baking bread" with e.g. "common-law rules around easements and rights-of-way in property disputes."

You pass a law saying that such and such existing laws remain in force temporarily and set up a commission to study them in detail, figure out what's actually important, and author new model laws as needed for the legislature to consider. This can be dialed from piecemeal replacement to a wholesale rewrite.

Something along these lines is how most modern states handled it in practice, if you look at e.g. former Warsaw Pact states.