I don't know why, sometimes useful and good things just don't get upvotes :/
Edit: My only thought is, there is already build in safari dev feature like yours so you should consider offer other things as well.
Yea Safari and Chrome responsive design modes are great. This just allows you to have many device sizes open at once without needing to open multiple browser windows.
It’s great for people doing QA on your website as well, saves a bunch of time not having to switch back and forth between browser sizes.