Traditionally CI systems only monitor the quality of the code in an integration branch, signalling when regressions occur. Human intervention is required for repairs.
As the number of developers working on the same branch increases the risk of breakages/blockages increases. Eventually a point is reached where on average by the time a breakage is fixed a new one appears—progress on the branch becoming effectively negligible.
Splitting into multiple teams each working on a separate integration branch which would be merged at a later moment might help, but greatly extends the project duration, since it simply delays the necessary integration for a later moment while adding churn/noise/technical dept related to the partial integration from individual branch merges. Costs also increase due to CI setups for each of the branches, each with their own build/QA resources, etc. And overall quality is not necessarily better.
Scalability is, at best, dubious.
Is there a method for making such large projects actually scale?