We had this with one of our cats and our first dog.
The cat was severely ill. His kidneys were dying. With the right medication, he could have lived for about three more years. But he was vomiting constantly, totally incontinent, and rapidly losing weight. The decision was made to euthanize him.
With our dog, he was showing clear signs of severe lethargy and he had this hacking wheeze related to an enlarged heart. He’d stand still for several minutes straight and not respond to us at all. We made an appointment to take him is for a check up, but we knew that a 13 year old cockapoo showing these signs meant the end was coming. We were prepared to euthanize him if it came down to it.
He died of a heart attack two days before his vet appointment.
Pets are not forever. They’re real creatures with a finite lifespan. It’s better for them to go out in peace than struggle until their body gives out.