What's the aim of a "good" adapted screenplay and how did All Quiet fail that?

I haven't seen any of the earlier or original material for All Quiet On the Western Front, but I've heard a lot of complaints about how the movie destroys the ending by changing it.

However, that leads me to this: does a good adapted screenplay change the original material to elevate it or is it faithful? The fact that the Netflix adaptation changed the ending doesn't make it a poor adaptation; that's like saying the Shining is one of the worst adapted screenplays. It does seem like the greatest critics of the movie are the ones most familiar with the older sources, but it still got a lot of love.

Seems like a screenplay that's less faithful would take more work and would thus be more deserving of acclaim rather than, say, just reading Shakespearean lines and following the play.

(I would personally take like Bones and All over this movie for screenplay but no one asks me.)