Would be pretty unintuitive, and completely bull with targeted dashes. It would feel very odd to use target dash on someone and then stop...in the middle? And what about the damage? It wouldn't make sense to get damage from a dash that doesnn't reach you, but that would be pretty OP.
And low-range dashes would be pretty screwed too; Yasuo now only moves a pixel. Blink users on the other hand don't care; Ez
Incidentally, there are also a number of 70% + slows out there, which would completely crap on any dashes. Suddenly, every character with slows would get a huuuge powerboost. Spammable slows would act like mini-silences.
Overall, it's too wonky and radical to implement. A more tame idea would be probably if slows would prevent movement spells for a tiny duration; not for the whole duration of the slow, else they'd be silences for mobility spells. Not sure if that'd be a good or an bad idea though; just tossing out.