One of the biggest reasons that Syndra is so good in the highest level of play compared to other mages is because she doesn’t have to stop moving while casting her DPS abilities.
Since making her stand still to cast Q after like 10 years of not having to do this would understandably cause massive dissatisfaction for mains, the better way to hit her mobility would be to bring her down to the lowest movement speed tier. For reference, this is what Orianna and Anivia have.
Syndra doesn’t have any movement speed buff in her kit, that nerf would essentially kill her unless her number are broken.