Could you flesh this out so I can understand the benefit? Are you replying to each of the threads, and thus need 5 compose windows as well? I tend to stick with windows and rarely use tabs, so I certainly could be missing an essential use case.
One tab: an email with a homework assignment.
One tab: my inbox.
One tab: composing a reply to a recruiter.
Two tabs: Other emails from the recruiter I'm using to compose my reply to the recruiter.
The first three I'm fine with being separate tabs in Chrome. That's fully intentional. However, the last three tabs are all one task, and in my mind should be achievable in one window/tab.