(Supply and demand I get, "force multiplier" I don't get.)
The other way you can go is a qualitative improvement, which is where the child mortality example comes in. If one engineer can remove 75% of the fuck-ups caused by manually performing a task, then that is also worth money, but it's not valued in quite the same way. You'd value lower child-mortality in person-years of economic output (accrued to society), quality-adjusted-life-years (accrued to the individual) and diminished malpractice suits/insurance (accrued to the hospital or the insurance company).
Perhaps a better term would be "productivity multiplier". Productivity is a pretty standard economic measure (average economic output per individual), and engineers both have much higher economic output per individual, and can greatly increase that output for other individuals as well (often with a one-time investment).
I kind of believe, I think not controversially, that compensation results from supply, demand, and a bunch of more or less arbitrary rules which either affect supply & demand or directly affect the price (price controls, visa regimes, etc.) Productivity (value created) sets an upper bound for compensation (well, actually, it doesn't, perceived productivity does; and even that isn't true - building a barely working website for the government for several hundreds of millions of dollars is one of the countless counter-examples), but productivity certainly does not set a lower bound for compensation - competition does. How much labor you save is beside the point.
Doctors have to do 50-80 hours per year of certified continuing education thingies. That's probably way less than the average software engineer, but at least now we have a concrete number :)