Sadly it is very common. I have lost count of the number of relatives who demand that their 95 yr old frail relative should spend their last moments having their ribs broken in a futile attempt to resuscitate them. They do not get to insist that we torture their relative.
We see similar in parents who cannot accept that their terminally ill child, who is in constant pain, so heavily drugged that they have little or no awareness of anything around them, will never leave an intensive care room and have no quality of life whatsoever, should be allowed to peacefully slip away.
Don't get me wrong - it's their child; if that were my child I'm sure I'd grasp at every straw, too - but that is why people who are emotionally in bits can't make these decisions.
To keep someone alive just because we can is not right, and it's not kind. A gentle death is actually a wonderful gift, even though many are too upset to recognise the fact.
It's not so long ago that illnesses like OP's uncle's meant an agonising last few days/hours - in many parts of the world they still do. Here we are fortunate; the situation can be carefully assessed (and they are - it's not random guesswork, and the option which is best for the patient is chosen.