But weren't medieval Christian attitudes towards usury a good thing?

Well, whether is was good isn't really a yes-or-no answer, especially when it got tied together with religious persecution.

I don't think I'd like to reintroduce it exactly as it was, but it's a useful example because:

1. It demonstrates how a strong economic-norm can arise from popular concern that a certain "buyer/borrower beware" arrangement is too prone to abuse.

2. Being unusual to us, it's easier to see and consider, as opposed to existing norms we may take for granted as "just how things are."

3. Since it changed over time (albeit by going away) it indicates that change is possible, and shows those economic rules are socially-constructed.

wouldn't treating lock-in as taboo be a good thing?

I guess usury is analogous to lock-in in your parent comment, not the Christian attitudes.

No, because who would lend money when they would just get the same amount back later, plus the risk of not getting repaid?

Pretty sure that's the point.