I've seen what happens when an org tried to adopt a Google-style monorepo without making the investments in build tooling and cultural change. It was a disaster.

All of those things are needed even at orgs much smaller than Google, or you will end up with an unbuildable, unmaintainable, unreleasable mess.

For orgs that can't make those investments, I think a repo per team is the best approach. Each team can treat their repo like their own little Google-style monorepo if they want to.