At the most basic level, Long Day's Journey Into Night is a play about people who are suffering. The characters have basically nothing to do, as the adult sons aren't working, the father is in his off-season, and the mother doesn't work at all. They tend to sit around, argue, and suffer the pain of old wounds and dark prospects for the future. One son has consumption to worry about, and the mother is addicted to morphine, but these problems radiate to the father and other son as well, as they suffer from the consequences too. Everyone's bogged down in depressive thinking about the way things used to be (either good or bad) and their (probably awful) future.
The kind of suffering we see on this one day's journey into night doesn't appear to be much more serious than the suffering that's probably been going on in the Tyrone family every day for at least 20 years.
Instead of criticizing her means of coping with pain, the Tyrones ought to look into a less extreme way of dealing with Mary's arthritis. Mary has a legitimately painful disease that is the impetus for her use of morphine, as her physical pain combines with the knotty reminders of her loss of youth, innocence, and beauty to send her into a tailspin.