I can't really speak to how Linux project works or how they organize their code. What I can tell you is that there has been research done that shows that TBD allows an organization to scale as their code velocity increases and the number of developers increases too. Great book that has results from a multi year study said this:
Forsgren PhD, Nicole. Accelerate: The Science of Lean Software and DevOps: Building and Scaling High Performing Technology Organizations (Kindle Locations 971-985). IT Revolution Press. Kindle Edition.
I can also attest to this from personal experience working on a large volunteer project with lots of developers, before starting TBD merging feature branches was stressful event and took hours to complete. We went from quarterly releases to weekly after we switched to TBD and other changes that where made to support that. We could not do that if we still had those long lived feature branches, and we no longer have to deal with the pain of merging large branches of code.
Adam, if you have any research that shows the opposite I would be certainly interested in reading it. At the end of the day its up to you to decide how you want to organize your projects. I know there is a time places for branches, but the trouble I have seen is the longer these branches live, the more pain they cause. There is a tipping point where any benefit to placing code in a feature branch is out weighed by pain it causes the team when they need to integrate that code back into the main line.
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