I'm not sure about "scale" but Linux, as an open-source project where external actors want their code included, can push arbitrary amounts of work onto those ac

I'm not sure about "scale" but Linux, as an open-source project where external actors want their code included, can push arbitrary amounts of work onto those actors with no additional cost to itself. They can say "to get your code upstream you have to do twice as much work" or 3x or 4x or whatever. It's not their cost to bear. An internal team pays that cost. They have to consider whether the trade off of having a pristine commit history is worth the additional overhead of doing it. I personally care more about PR size and am happy to squash all commits in a PR. If that is too big I'd rather see multiple smaller PRs.