Right. Put differently: any sufficiently long-lived codebase contains institutional knowledge. Knowledge that goes deeper than just the explicit interface bound

Right. Put differently: any sufficiently long-lived codebase contains institutional knowledge. Knowledge that goes deeper than just the explicit interface boundaries that you're planning to re-implement. That doesn't mean rewrites are a bad idea, just a disruptive one. They'll introduce new fragility for a while before they start being a net gain. Code has to be broken-in, like a new pair of shoes.