Yeah I think what you are suggesting might work if team follows * merge PR asap * keep build time down to n mins.
But its extremely common in big projects to have atleast 20 prs open with long build.
kubernetes has like 1100 open prs atm ( each pr with 5 25 min builds). What should a project like k8s do for CI ? would require insane computing power to launch 5000 builds for every merge.