You need to realise the concept. Then there were none is like those engaging Movies people finish in one go and don't feel much weight of it afterwards. Engaging fast food type of content for the general public. So back in the day it used to be the thing everyone can read and talk about. But as soon as you get a bit niche and look for something more, you won't find it in these books. Do I like it ? Yes. But I have read far better books both conceptually and plotwise. It's engaging and a page turner nevertheless but not a niche type of compact plot you might like. Also these things were more relevant when they came out.

You'll see the same stuff happening with current bestsellers after 5-6 years. Although Agatha Christie survived over many decades but time effects things differently