I would consider it unethical to try and mislead the manufacturer into repairing the knife by claiming it is defective if it is not defective.
However, if you told the manufacturer the truth, that the knife has served you well for years, that it has been a valuable tool and cherished companion, that it has worn out from extensive use, and you ask them if they offer any free refurbishing services, I would see nothing wrong with that.
Who knows, maybe the manufacturer would refurbish it for free, in appreciation for a customer who values their product. Perhaps they would consider it good PR.