When I read the white papers by google or uber on their monorepos, when I see what my company is building, it is just a custom VCS. Everything that was thrown away initially gets rebuilt over time. A way to identify subprojects/subrepositories. A way to check out or index a limited number of subprojects/subrepositories. A way to define ownership over that subproject/subrepository. A way for that subproject/subrepository to define its own CI. A way to only build and deploy a subproject/subrepository. Custom build systems. Custom IDEs.
The entirety of code on the planet is a polyrepo and we don't have problems dealing with that scale like we would have if we stuffed it all in one repo like this debian monorepo shows. Independence of lifecycle is important, and as a monorepo scales up people rediscover that importance bit by bit.