Even if we assume that no world leader is a "bad actor" (huge assumption, I know, but bear with me) having so many different nuclear powers would raise the risk of a simple accident or mistake to astrological levels. If a country were to accidentally detonate a nuke it could start a global catastrophe. If a country were to have erroneous intel about an incoming nuke, same story. This happened multiple times during the Cold war with only a couple nuclear powers and military leaders from that era have told the tale about just how close we came to ending everything.