It’s supposed to be less militaristic. By the time of tos Starfleet is pretty well established as being a space navy. The idea was that in the beginning Starfleet would be more like a band of explorers who were all experts in their own fields and everything, including the command structure would be less formal.
$100k is about the average household income in the US, the average single income is about $67k. So about a year and a half of the average wage. And median income is much lower (median household income is about $44k) indicating that high income households have a much larger effect on the average than low income households so the average middle class individual income should also be much lower than the average individual income.