I am fond of pugs. I've had three. The first I had to put to sleep, the second went to live some other people because he was an outdoor dog and preferred dogs to people, and the third got old and sick and I had to put him to sleep Monday.
He went downhill in five days. He quit eating on Saturday, and by Monday I realized his time was up. I made an appointment for Wednesday.
By Monday he was in horrible shape - and it only took an hour. His breathing was very labored. Among humans it would be Cheyne-Stokes breathing. When I checked his eyes the surface was peeling off.
That was it. I found a place four blocks away that said bring him in.
I walked in with my girlfriend, with him in my arms, and the assistant took him immediately to the vet. She came out five minutes later and said he immediately "went to sleep."
They put him in the box I had bought, we left, and I buried him in the backyard.
He was about 13 years old. I'm not exactly sure because he was a rescue pug, and was supposedly seven when I got him.
If you are not willing to deal with an old, sick dog and put him out of his misery, then you should not have a dog. Because doing this to such a dog, as painful as it is, is a mercy to them.
Rudyard Kipling understood these things, with his poem, "The Power of the Dog."