My understanding from many Google employees is that the properties of the system that caused problems in ~2000 - 2010 are largely still the same today: the canary node model of deployment, fixed small set of supported languages, code bloat, inability to delete code, bias towards feature toggles even when separate library dependency management would be better for the problem at hand, various firefighting when in-house monorepo tooling breaks, difficult on-boarding for people unfamiliar with that workflow, difficult recruiting for candidates who refuse to join if they have to work under the limits of a monorepo like that.