In the modern age, paying customers normally get the opportunity to choose. When forcing through a UI change like this, admittedly "difficult" to get used to, simply for your app to look more like your other apps, it begs the question, why should the outlook app UI change, and not your other apps UI?
Outlook must be one of your more widely used apps, in the MS family of office apps, and why mess with perfection? Since Windows 98, MS have become more and more controlling, leaving users with less and less control, and less and less choice. Anything used to be possible, and these days nothing seems to be possible. Normally "security" is quoted as the biggest single reason for these draconian forced changes. But we the USERS are paying customers, and WE don't like being dictated to. Even by MS. Even if you decide that it's better for US. Because We know what we like, and We should retain control over our hardware, and not be forced to cede control by simply choosing MS office apps.