I haven't read the thread so forgive me if I'm repeating what someone has already said, but the exciting thing about Kyrgios--which is more relevant now, after we've seen him beat Novak twice--is that he's the only young player who seems to have a chance of actually wrestling away the crown from the current elite, rather than just getting it by default because the old guard is aging. Clearly Kei, Milos and Grigor aren't going to do it, or at least haven't done it yet (there's been signs of life with Grigor). Zverev and his cohort are still a couple years from playing at a peak level, at which time the Big Four will all be deep into their 30s so clearly in decline, but Kyrgios is capable of beating anyone right now - even when the Big Four are all close enough to peak level.
I could see Thiem being a serious threat to the Big Four during clay season, so maybe he'll join Kyrgios as a "giant-killer" in that regard.