I remember seeing the crash test for the Rover 100 and that killed rover sales overnight with it's dreadful rating.

Yet these cars were sold for nearly 20 years and people were happy with them.

Does this mean that a car brought now that we think is safe turns out to be a future death trap ?

At the point in time when you buy a 5* car it has been shown to be demonstrably safer than a 3* car. In two years time it will still be safer than the 3* car.

Only if the testing standards aren't raised or manufacturers don't raise their own safety features. You have to compare aples with apples, i.e. cars tested at the same time under the same testing regime.

My 14yo car is a 4 star rated car. It would likely be only a 2 star car today, perhaps a 3 or 1.

Note also that unless a car material changes through a more comprehensive makeover in mid-life, a car that has been on sale for 7 years is more likely to be less safe than one that has just come out because the star rating is based on when the model was originally tested at year zero.