If you are searching through e-mails trying to find the one you are looking for and trying out different search terms, it used to be convenient to type in a search term, click through the e-mails to see if the correct e-mail appeared, then click back on the bar to try a new search term. Now with the new location, the bar as both further away from the results (especially if using the full ribbon) AND smaller, making it a harder target to click on. When you are rapidly going back and forth, this makes the process of identifying e-mails more cumbersome. Search is probably the most widely used feature of outlook. WHY REDUCE THE SIZE OF THE CORE FEATURE AND MOVE IT TO A PERIPHERAL LOCATION?? The shortcut Windows+E was introduced to move the cursor to the new bar, and this works well when typing, but if you want to use the mouse to scroll through and click on the search results, then you would need to switch back and forth between keyboard and mouse. Based on this, this new search bar is decent when using the keyboard only but un-necessarily cumbersome for mouse navigation.
Based on all of this, I conclude that one of the following likely motivated this shift from Microsoft:
A. Microsoft has shifted it's priority on the Microsoft365 webmail. The Outlook desktop application was redesigned to look more like the Outlook webmail for ease of development, at the expense of actual usability of the desktop app.
B. Microsoft is trying to push it's "AI/Machine Learning": "see, the search bar UX doesn't even matter because our nifty algorithms will identify the right message on the first try". LOL
Assuming it's A, this is a slap in the face to all users of the desktop application.