That is certainly not the normal flow. I am not sure of that is a real-world example (can you provide a link to a PR?) or just illustrating.
I can’t speak for all ROS repos but in ros_comm any PRs are only merged into the latest devel branch. The only very rare exception is if a patch is only necessary for an older distro but not the latest one.
And then in a second step all changes on the latest devel branch are being considered for backport to the previous devel branch (and then from there to the second previous etc.). See changes between 1.12.12 and 1.13.6 for backporting by dirk-thomas · Pull Request #1323 · ros/ros_comm · GitHub for an example of such a backport.
So as far as I understand you proposal that is exactly what is happening in ros_comm with the only exception that the “single line of development” is not named master but after the latest ROS distro.
I would suggest to continue this specific conversation in a separate topic / thread since it gets a little bit away form the scope of the original topic.