An unspoken truth of a monorepo is that everyone is committed to developing on trunk, and trunk is never allowed to be broken. The consequence of this is that execution must be configurable at runtime: feature flags and configuration options with old and new code alongside each other.
You can have a monorepo and still fail if every team works on their own branch and then attempts to integrate into trunk the week before your quarterly release process begins.
You can fail if a core team builds a brand new version of the product on master with all new tests such that everything is green on every commit but your code is unreleasable because customers aren’t ready for v2 and you need to keep that v1 compatability around.
You can have a monorepo and still fail if every team works on their own branch and then attempts to integrate into trunk the week before your quarterly release process begins.
You can fail if a core team builds a brand new version of the product on master with all new tests such that everything is green on every commit but your code is unreleasable because customers aren’t ready for v2 and you need to keep that v1 compatability around.