Each ecosystem around a set of technologies essentially has its own "Herfindahl index"[1] of sorts. For example, the Ruby ecosystem's index might be fairly high, simply because Rails is so dominant (to the point where Ruby and Rails are often conflated by outsiders). On the other end of the spectrum, the current JavaScript landscape seems to have a fairly low index; there are a huge number of tools with very few clear winners at this point in time.
I suspect the best/healthiest ecosystem is probably somewhere in between that of Ruby and JavaScript. Rails may have ultimately hurt Ruby by its dominance, just as the current volatility in JavaScript tools seems to drive some people away (or at least add frustration).
Of course, I have no numbers to back any of this up - it's just an observation (one skewed by personal experience, no doubt).