Standard of living is a function of wealth, but wealth is not the sole factor. It also includes quality of the environment, crime levels, access to good medical care, education standards, civil rights, house affordability and so on.

Most people nowadays are poorer than Mr Darcy in a monetary sense, but probably have a higher standard of living.

The sibling post makes the point that in the past, being wealthy was a much bigger component of this - if you're rich you can afford a doctor (versus none), you can eat when others go hungry, you can afford to travel at all, you can be educated, etc.

As I understand it, standard of living is objective - you can quantify it fairly well. Quality of life is subjective.