Stability & predictability of your web presence is more important than the "perfect" web page. Don't make big changes, only little ones.
If you went into your shop or kitchen & someone had completely rearranged things you would be upset, especially if it happened spontaneously with no notice. If the dashboard of your car was completely changed at random times you would be an inefficient and even dangerous driver. The same is true of any website. Stability of the website is more important than the "perfect" site because "perfect" is different for each person. If changes must be made make them a little at a time so people can get used to them. Yahoo email is a tool that people depend on. Just like I want my hammer to work the same every time I use it the same is true of my email. I don't suddenly want a red hammer or a pink one. I want one I can depend on that works the same every time I need it.