Realism should not be used in this argument because the same wear and tear argument could be used against items you've had forever but not heirloomed yet. Everything already breaks and repairs as it is.
And even if you insist on realism, you could still turn it against the argument saying: The weapon turned out to be well-made, better than your average sword, and that is why it was chosen to pass down to your heir.
All that should matter is the balance of gold repairs, not realism. So long as the reductions in break chance were not drastic, I'd personally see no problem with it.