+1000
It becomes very confusing very soon if you can rearrange modules independently of pipeline order.
Concrete example. You notice some color artifacts. Where is it originating from? You start disabling modules from the top, until they go away. You found the source of the problem.
If the order were arbitrary you would have to know the actual pipeline order by heart.
You may ask, why not adding an option to make this possible for those who want to? I would argue against it. darktable’s interface is already very complex, if anything it would need to be simplified.
For example, I would advocate that most of the predefined module groups are not necessary. I would only keep all, all active and user-defined groups.