Thoughts on putting down elderly pets?

I am once again dumping my raw thoughts on Lemmy and asking your opinion on them.

My first dog (and pet in general) is nowhere near the age of me needing to think about putting her down, but having a dog has introduced me to the world of opinions on whether they should be put down when they get too old.

I've read a lot of very strong pro-euthanasia pet owner opinions, even going as far as accusing people refusing to put down their pets as "cruel" or actively wanting their pets to suffer. It really seems like a majority of pet owners, at least in the English speaking world, think putting their pets down is something you should always do when their bodies deteriorate past a certain point, and every time this is brought up you get a lot of emotional comments shaming anyone who doesn't subscribe to that philosophy.

The core argument being made seems to be that when their health conditions pile up past a point, it's not "worth" letting the pet live anymore, supposedly for their sake. But when I think about it further, I ask how can you be sure? All animals want to keep living, that's literally why animals evolved brains in the first place, to keep their bodies alive for as long as possible. How can you, who is not the pet, say for sure they would prefer to die than keep living? You can't ask them, and you can't get in their mind to determine how much they still appreciate being alive. Even the oldest, sickest pet will still make an effort to keep themselves alive however they can: eating, drinking water, moving out of the way of danger, etc. As far as I know, no animal (at least the animals we keep as pets) have an instinct to just give up and stop going through the motions of life past a certain age. Doesn't that imply they always want to live?

I consider the decision to no longer live past a certain age and certain number of health problems to be a uniquely human thing, and it doesn't feel right to impose that on a pet who probably doesn't have those thoughts. Even with humans, we refrain from making that decision for them. Someone who's in a coma isn't eligible for euthanasia just because they haven't expressed a desire to live, and the most their family can legally do is to stop actively keeping them alive with technology and let them die naturally. But if they don't die right after taking them off life support, you can't just straight up kill them, they need to die by themselves. Why isn't this philosophy applied to pets, who can never consent to euthanasia? You don't have to keep subjecting your pet to more and more invasive treatments just to extend their lives by a small amount, but at the same time, what gives you the moral right to unilaterally decide when they're done with living? Why is letting your pet die naturally in the comfort of their own home seen as cruel, while choosing for them when they should die is considered humane?

What do you think? I genuinely don't know how I feel about this but want to understand the problem and where I stand on it before my dog gets old enough for these things to apply.