I definitely disagree that adapting to 30fps is harder than adapting to lower quality shadows, in terms of consistency.
My point is that if the game holds a constant 30, there is nothing out of place. The game runs at a consistent resolution, the shadows stay a relatively consistent quality throughout, and there aren't those additional technical hiccups that interrupt during gameplay and take your attention away from the gameplay or action at hand.
In 60fps mode, there is absolutely that huge benefit of increased framerate, but along with that comes some technical quirks like missing shadows, and lower resolution shadows. That alone wouldn't really be too noticeable, if it wasn't accompanied by much higher quality shadows (the prebaked ones) alongside it. It means that the player will see these crisp, high quality shadows directly alongside these pixellated, shimmery ones and that is inconsistent with the rest of the world. Because it's inconsistent it draws your attention and you'll notice it more regularly.
Playing in a lower frame rate (for most people) is something they will be able to consciously acknowledge once they start playing, but if it's solid never have to think about again.