Windows 10 Insiders running the latest preview build of Windows 10 (Build 10532) have a new option in the Windows 10 Mail client: They can turn off Conversation View.
Woooohoooooo! Thanks for the good news, @StephenCWLL.
My only regret is so far this option is not available to those of us running the RTM build of Windows 10. But hopefully we will get it soon, given it seems to have been part of the latest update to the Mail client for Insiders. (Because there is no itemized list of what's new in these app updates, few probably realize this feature is now there.)
Yes, I am a Conversation View hater. And I am not alone.