Well I think the issue is not, would you prolong the life of the pet even though quality of life was very poor... but rather if the treatment would FIX the pet, would you drop a unending amount of money into doing so?
While a lot of us would say yes, or no.. you don't know until it happens(asin most theorectical discussions). I will not endanger my home to fix a pet. I won't begger myself, and be unable to pay my bills, to fix a pet. I will spend whatever money I CAN to fix a pet, and if the vet will allow payments, I will run up a REASONABLE bill to fix the pet.
I think that most of us differ merely in what we consider reasonable. I would hope that no parent would take food from their child's mouth to fix a pet. I will discontinue luxarys(i.e. tv, internet, etc) in order to pay for a pet's care. I won't turn off my electric. And again.. there's a limit to me. I rarely take MYSELF to a doctor over anything. Superglue and ibuprofin work on me for most injurys.