Don't forget that houses get torn down and rebuilt. It doesn't happen often but it happens.

In the US, modern houses tend to be larger, have better insulation (from what I've experienced), but can be cheap in some aspects (e.g. interior doors tend to be hollow, whereas older houses typically had solid doors). Overall though, I'm not sure if new or older homes were better built. I know some really well built old homes, but some very poor built ones too, and this goes for modern homes as well. Most should last 50 years though.