3-10x is, of course, intended as a 95%-of-cases interval with some basic assumptions. Some contexts (e.g. type of problem, engineer, timeframe) will be below 3x and others will be above 10x. (The 99.99% interval is probably 0.2-100x.) Among those basic assumptions is that the engineers have the curiosity, drive, and ability to learn it. Ability is the least issue of the three; if they're not smart enough to learn Clojure, they're not smart enough to Java or C++
well, and they're mostly making messes.
Lisp has a bad reputation compared to what it is. People think it's impractical, that Lispers are arrogant (due to a few vocal but influential people in the Lisp community) and that it's archaic or weird. I've addressed those perceptions, but they're out there. Clojure is, in fact, a pretty damn practical language.
Now, the arbitrage argument: in larger companies, the concerns aren't engineer productivity so much as appearances. For a middle-manager to launch a meaningful Clojure/Lisp initiative would be to put himself out there in a major way, and since programmer productivity is only one piece of a much larger system, it might not have that big an impact on the company as a whole. It might not be worth it. (Would Google's market cap go up 3-10x if it replaced Java with Clojure? Almost certainly not. Maybe the fair value would improve by 1.05x on fundamentals, which would be drowned out by month-to-month volatility.) On the other hand, startups generally can't afford (or don't think they can afford) to experiment with newer, quirkier languages. VC-funded startups tend (surprisingly?) to have stodgy closed-allocation work cultures and boring tech stacks because they're expected to allocate 100% of the "risk allowance" to one department: business-model risk. That's not to say that there aren't other kinds of companies that can (and, in some cases, do) use more innovative tools; but the two dominant cultural forces (VC startups and large corporations) in technology don't favor it.
For the large companies, I'll admit that there's an Amdahl's Law sort of effect at play. Let's say that you improve productivity of engineers by 3-10x. Does that "3x" the whole company? As I alluded to for Google, probably not. It makes something else the limiting factor. For most firms, using the wrong programming languages is nowhere near their top problem. To use Google as the example, it might deserve a 5% bump on market cap if it included Clojure (a small gain compared to monthly volatility, so small that no one could prove causality and take credit) on the white-list but the fair value would go up 50-200% (again, on fundamentals, as I can't predict investor opinion) if it implemented open allocation. Programming language innovation just isn't the top priority for most companies. But for an individual developer, the advantage conferred by a powerful language can be pretty serious.