Preventative maintenance is very important in projects, which is why there is a huge emphasis on risk analysis. I see your point that task scheduling can create risk as well since each task can ultimately severely delay the project. Some project planning softwares include features that incorporate employee’s 8 hour work days, vacations, and other personal delays. Doing so would allow for a more accurate gant chart. Risks with the ES, EF, LS, and LF dates can be minimized by very well planning. There are many other ways to minimize these risks, as others can elaborate more on.