It's very much the same idea as "don't start ripping out and refactoring 'legacy' code on your first day on the job." You may actually have inherited a completely garbage code base, but you may also simply not understand the constraints that dictated the architecture you've been given. Once you understand those constraints, refactor away!
With regards to your example: Dietary laws may well be outdated, but modern Jews may also argue that setting themselves apart is still a very worthwhile reason to avoid pork—it's probably part of the reason why they managed to remain a separate culture even without a proper homeland, and remaining a distinct culture is valuable to them.
Take cigarettes for instance. We know they’re unhealthy. There are also still millions (hundreds?) smoking them every day.
People give up and fall into habits unless there is strong reinforcement. If you get a parasite and have no way to get rid of it (because of lack of modern medicine) because you ate pork, not like you’re going to stop eating pork necessarily any more. Especially if it’s cheap in your area.
If there is another group of people who manages to not have those issues because they don’t eat pork - they’re going to keep not eating it after seeing that too!
See: dietary laws, circumcision, restrictions on what materials your clothes may be made out of, rules for what hair you're allowed to cut, and when you should or shouldn't cover your head, etc. etc.
(And before people jump in with "maybe there was a specific reason for the dietary laws, e.g. pork was unhealthy," that doesn't explain why you'd have peoples living side-by-side with opposing dietary laws.)