> Why can't we add millions of dollars of tool engineering on top of polyrepos

I don't think the "stock polyrepo" characterization is apt. Organizations using polyrepos already do invest that kind of money. Unfortunately, this effort is not visible because it's spread out across repos and every team does their own thing. So then people erroneously conclude that monorepos are much more expensive. Like the GP said:

> Polyrepos are a thousand species living and dying, some thriving, some never to be known, most completely in the dark.