Lack of browser diversity is bad because it puts too much control in the hands of the only browser vendor.
Lack of browser choice is bad because it puts too much control in the hands of whatever browser vendor they use.
We need both for a healthy web.
[0] Imagine that everyone was mandated to use one of five different browser engines, such that each had exactly 20% market share.
[1] Imagine that you are free to choose between five different browser engines but everyone else picked #3 and websites are only ever tested on #3 and all the others don't work so well.