The general warning of "correlation not causation" applies here. The studies show some correlation between sitting for hours and various maladies. However, there are so many factors that determine your susceptibility to the health issues you mention that calling out just one factor (sitting) needs to be viewed with some skepticism.
That said, I do use a standing desk for about half of my working hours because I think it is healthier to not sit all day.
There have been countless studies done at this point, spanning decades, directly linking severe health effects to sitting down. Particularly radical increases in the risk of heart attack and other severe cardiovascular trauma.
How many medical studies do you need? There seems to be a new study out monthly at this point proving the link between sitting down 8 hours per day and radically increased risk of death.
Unless you think the people sitting down all happen to be the ones carrying an inherently heightened risk of heart attack. See the 1953 British study I referenced above as a simple example. Recent modern studies have found the same results: sitting all day effectively doubles your risk of dying from a heart attack.
That said, I do use a standing desk for about half of my working hours because I think it is healthier to not sit all day.