The diversity that's important isn't in the number of companies using the same engine, it's in having several different engines.
Especially with the auto-updating of stuff these days even without users wanting it.
Diversity means competition. That means there's stronger incentive to improve the engine. It means malware has a harder time taking hold (though the google malware is everywhere anyway :( ).
It also means we users get a variety of choices and we can try something different from time to time.
If you want one model or brand of everything, go back to the 1950's-80's Russia (as portrayed in American movies).