To emphasize what wcoenen said: the appearance of spiral arms persists even in the interior of galaxies where there is no discrepancy from Kepler's laws. Moreo

To emphasize what wcoenen said: the appearance of spiral arms persists even in the interior of galaxies where there is no discrepancy from Kepler's laws. Moreover, in order to explain fixed spiral structure using modified orbital mechanics, you would need to assert that outers stars have larger velocities than inner ones (in order that their angular speed is the same, to preserve disk structure), but in fact the rotation-curve conundrum is based on the observation that outer star have roughly constant velocity, and so reduced angular speed.