A day of sorrow indeed... No, joking aside. I gather most people use double click anyway, so this is a good change for that reason.
I've never really understood it myself (the primary function of the left click being "select" when everywhere else it's "open" or "go to this thing"?? Alien stuff).
I'm just glad KDE listens to it's users and adapts to them. Looking forward to the release!
Its only open on things not meant for file management like web browsers, or when people use their monkey extensions as input device, instead of a mouse with 3 buttons and a scroll wheel ;D