Definitely a bad idea in my opinion! Putting theme options into the customizer did make sense. Although, now adding a theme is an extra effort with no easy link to the menu since the Navbar also replaced the old link!

Now, if the idea is to put Menu creation in there, then it's clearly not a good step for making WordPress user friendly. Menu creation needs to sit where it is. The interface gives you the space to work on it and create the menu. Putting it into the customizer is just stuffing in way too much in there and it's already crowded with all the existing options.