This is so important. It's about quality of life. When that's gone, it's time. If the vet is any good, they'll answer this for you: do they think the pet is suffering? It can be really helpful in alleviating guilt that it was "too soon."
Where I differ from your perspective is that I will always choose "a little soon" over "a little late." This is borne out of personal experiences - watching a pet obviously suffer until we could get a vet appointment is among the most traumatizing thing I've gone through. Even one day can be too much. For us, it's cats, and they can go downhill pretty quickly, so for dogs it might be different.