Then all we need to do is either spin the earth faster to make a year 400 days, or add enough mass to scoot the orbit further out so a year is 400 days. The each season can be 100 days.

Is it really easier to do it one way or the other? If you know one way–and keeping time is one of those things pretty much the whole world does the same way–learning something new isn’t going to be as easy. So why even bother switching?