Another way to look at it is that maybe it's because of economic development per se? Inflation adjusted incomes look fine but real per capita incomes has grown

Another way to look at it is that maybe it's because of economic development per se? Inflation adjusted incomes look fine but real per capita incomes has grown since 1919 by about 7x. So the social-ranking equivalent of their salary wasn't $50K but about $350K, they were in ~top 5%, and maid's salary "social" equivalent was about on the level of today's minimum wage. It is quite fitting that top 5% of families can afford maids earning a minimum wage. In particular case of UK, nominal GDP per capita was 119.05 pounds in 1919 so maid was making 36 pounds = 0.3x of it. Now nominal GDP per capita in US is $70K and it's completely OK if the maid makes $21K, that's even well above minimum wage. Surely one can be hired for the amount.