Sitting is natural and not “bad”, by itself in moderate doses.
Sitting is an incredibly natural thing to do. True we are able to sit way more than we ever really evolved to do, but that isn’t to say we didn’t evolve to sit. People in hunter gathering tribes also spend a large proportion of their day sitting, upwards of 6+ hours a day. The modern office worker probably spends somewhere between 8 to 11 hours sitting, but those numbers vary widely from person to person and for a single person over time. In addition to us nowadays sitting slightly more, we also tend to do so in longer stretches without taking a break to walk do some physical activity. Also most of us sit in chairs with back supports, which allows our backs to atrophy somewhat. Hunter gatherers roamed around and didn’t bother making furniture. They would sit on the ground. The hunter-gatherer sitting pose of choice seems to have varied between groups. Some had their feet out in front of them, some off to the side.