Hrmmm, when you said all the top players have had a weapon I was thrown, but then was corrected when you said top 5.....She doesn't have a consistent weapon. Every top player of the past many, many years has a consistent weapon. Her serve and movement are good but not really noteworthy, neither one of her groundstrokes is reliable enough to lean on in a match... what is she going to do in a tight situation to get past the players she needs to?
The big thing in my mind is that she didn't develop properly. She hasn't played tennis competitively consistently in her entire life. Even when Venus and Serena boycotted the junior circuit, they had consistent hitting partners and training. It's unclear that Na/Li ever had that sort of consistency in her development.
Plus, there have always been question marks about Na/Li's ability to make it. She emerged waaaaaay later than pretty much every slam champion or top 5 player ever has. And her emergence was...well, what was it exactly? She never hit the top 10, never won a Tier I or II, never made the semifinal of a slam, only has one slam quarterfinal, only won multiple matches at a slam 8 times... that's a pretty paltry career for a 27 year old that people expect to be a top player. And that's why those of us who have kept our eyes open have been pretty sure she never will be a top player, because the signs just aren't there.