> There is no easy way to determine "the ROI of what the compensation could otherwise be invested in",
That's exactly what cost accountants do. A large part of running a company successfully is determining the ROI of various uses of capital. The people running companies are not idiots.
It seems like you're saying that the way salary negotiation works is: the manager benchmarks the candidate's salary request against an alternative investment, like bonds, &c -- not an alternative candidate in the market place -- and then considers whether to spend that money on the candidate or on the investment.
That's exactly what cost accountants do. A large part of running a company successfully is determining the ROI of various uses of capital. The people running companies are not idiots.
There's even a word for it - "opportunity cost".
https://www.investopedia.com/terms/o/opportunitycost.asp