As you yourself pointed out, trying to squeeze all aspects of how well people are doing into one metric is tricky - although useful efforts in that direction exist, eg Human Development Index [0]. That's why we're focusing on one specific aspect here, economic activity, for which flow-based metrics like GDP or household income are better suited than stock-based metrics like net worth.

[0] https://hdr.undp.org/data-center/human-development-index