It's pretty well hidden, but you can still compose messages in a new window. For new messages, hold down Shift when clicking on the Compose button, or just hit Shift-C (but only if you have vim-style keyboard shortcuts turned on[1]). For replies, you have to click "pop out reply" first and then hold Shift while clicking the (horribly labeled) middle button in the resulting reply pane / thing[2].

...and the fact that I had to write this comment proves your point.

[1] https://support.google.com/mail/answer/6594?hl=en

[2] http://i.imgur.com/lWXEP1Y.png