People who've never had to make the decision to euthanize a pet - one of the hardest decisions you'll ever have to make, and one you'll likely need to make many, many times throughout your life - don't understand what it's like. They've never spent thousands of dollars, months of fending off the grim reaper, and numerous sleepless nights growing increasingly aware that you're about to acquire some deep emotional scars.
It's easy to say "I'd do anything to keep my pet alive" as a purely hypothetical scenario, but reality is that at a certain point there's nothing anyone can do. Mortality is as mortality does. Fighting it just makes you exhausted and broke.