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?