I respect your assertion, but I've heard this argument before.
"It gives you an 'infinite' Y target...you just slam your pointer into the top of the bar and get menu options"
Except that essentially nobody does this. Remember that you and I and other HN readers are not the general public, and the general public is not going to assume they can do this. Watch actual users. Not to mention that this only helps in the Y direction; you still have to position carefully to get to the horizontal item. Most people do not separate those two. I've been programming on Windows and Mac for decades, and I never ever "slam" the cursor to the top of the screen.
Also, "slamming" the cursor in Y will not work if you're using multiple monitors and one is above the other. For example, if you have a laptop on your desk with a big monitor above and behind it.
And any loss of "slamming" convenience is offset by the proximity advantage of a window-mounted menu. If the menu is on the window you're working in, you have to move the cursor less to get to it. On today's large, high-resolution monitors this can be quite a savings.
And finally, with the menu on an app's main window there's never any confusion as to which application's menu you're using. Again, with today's large screens, you are more likely to have multiple applications' windows up. It's hard enough to tell which application owns the menu without peering at the corner and reading a name, which is never a fast or intuitive solution and even worse if applications are presenting similar UIs (for example two graphics-editing apps).
And if you minimize an app and are left with a couple more on the screen... who owns the menu? Who remembers the last place he clicked? Why should you have to click around on apps' windows to swap the menu out to the one you want, and then roll all the way to the top of your 27-inch screen to use it?