It's my understanding that you aren't obligated to accept luggage replacements the airline suggests. Instead, you just need to purchase a new suitcase and send the airline the bill for it pursuant to Article 19 of the Montreal Convention which applies to all travel domestic and international when it comes to lost/damaged/delayed baggage. The amount they have to reimburse you is capped at around $2,000 USD per passenger but I suspect your luggage would cost less than that. The important thing is to find something comparable to what you already had before (doesn't necessarily need to be an exact match if they aren't making that bag anymore). Also you don't need to have your original receipt for the luggage you may have purchased years ago, all you need is the receipt of the new luggage since by law they have to reimburse you for the replacement cost not what it may have costs you several years ago. And no, the fact that you used the bag for some time doesn't diminish the amount that they must reimburse you. Keep in mind Article 26 of the Montreal Convention prohibits the airlines from coming up with their own policies to limit liability aside from what is stated in Article 19 of the Convention. This is the bargain the airlines take. They get predictable and reasonable liability caps in exchange for not creating their own rules.

-RooFlyer88