• in the second scenario, I might have not changed the results of the step, but my refactoring changed the speed considerably and this is something I measure.
• in the third scenario, I might have not changed the results of the step and my refactoring just cleaned the code, but besides that, nothing substantially was changed. Thus I do not want a rerun.Well, I would say then that in the second scenario it’s just rerunning the pipeline, and in the third it’s not running it at all 😄
  (I mean, in both the code may have changed, the only difference is that in the second case you’re interested in measuring it, and in the third, you’re not, so it sounds like a user-specific decision).

At any case, while I understand now what Martin meant, I still feel the function-based pipelines are the strongest option, because it sounds like you’re looking for a way to dynamically build your pipeline.