I disagree with this. The recipient will know they have received slowed funds and can value them accordingly. It is up to the recipient to guage the risk of fraud and deal with the risk accordingly. Slowed funds would probably be valued at a discount coresponding to how much it would cost to insure the risk of fraud. If a person only has slowed funds and the recipient won’t accept them, the person can trade slowed funds for unslowed funds from one of his friends or somebody who trusts him. There could even be professional dealers who buy slowed funds at a discount.
If somebody intended to commit fraud, and were able to do so profitably while still paying the penalty, then the penalty would still not stop fraud. It would only reduce the proceeds.
Even the presence of a penalty would not cause slowed funds to be valued the same as unslowed funds. The would still be a risk discount.
The penalty might reduce the proceeds for some intentional fraud, but it would certainly hurt honest people who need to use the feature. That would be unjust. If you are okay with such injustice based on the logic that the person should have taken more care with their keys, and therefore there should be a penalty to deter carelessness with keys, then there is no difference in principle with letting the person lose all their money. They should have taken better care of their keys, right? So 100% loss is fine too based on that logic, and provides an even better deterrent against carelessness. May as well not have the feature at all if you want to deter carelessness with keys. If you care about justice, then justice needs to be 100%, not 95% or 90%.
Who would recieve the proceeds of some honest person’s mistake or misfortune? The miner of the block? The founders? I do not think miners or founders should get a boon from somebody else’s mistake or misfortune. The transaction will have a fee. That should be good enough.