Again, I think you're tangling together cases. And also tangling arguments. You can't argue from theoretical gains in an imagined optimal case and then say the only argument that matters is the empirical one.

If your point is that in some existing organizations stacked pull requests are better for a specific engineer's experience than doing all the related code in a big blob, I certainly believe you.

Similarly, there are manufacturing shops where reducing inventory in line with Lean approaches doesn't work out of the gate, because there are other organizational problems/constraints that have to be dealt with first. For example, you might need a large buffer of component X at stage Y of a manufacturing process because upstream quality issues mean that a smaller buffer would cause frequent stalls at stage Y. First you have to fix the upstream issue before you can cut stocks there.

So are stacked pull requests the optimal choice for some specific person on some specific occasion? Sure! I'll take your word that's the case for you. What I'm saying is that I think they're an indicator that there is some systemic problem that could be resolved so stacked pull requests and giant pull requests both become unnecessary.