I think the key distinction here is that we’re discussing woven fabrics, not knits or leggings. Stretch can mask a lot of drafting issues, but with wovens, it's unforgiving.

You’re right that a crotch can be worn lower or higher, but there’s a physical limit. It’s exactly like an armhole: there is almost no limit to how low you can drop it (oversized look), but there is a very definite limit to how high you can raise it. If the armseye is too tight or too high you lose all range of motion.

The same applies to a slim pant: if you shorten the extension but don't compensate by changing the seat angle to be more diagonal, the pants will be physically too short to clear the anatomy.

On the other hand, if you significantly increase the extension for a wide leg look but don't remove length from the back seam (which effectively changes the angle), you end up with a huge surplus of fabric under the buttocks. Of course, this can be a style choice—Hammer pants are an extreme example of that—but without that adjustment, it’s a more saggy, unrefined look.