A detoriating body isn't really a fall-back in a fight.
We saw how fast Palps moved in Ep. 3 or Yoda in Ep. 2 and Ep. 3.
Age means nothing in a force-wielders fight.
Did we see Yoda's body suffer massive deterioration from massive force feats, no we did not. There is a reason for this, while Palpatine was still able to augment himself to move incredibly quickly in Episode III, that didn't change the fact his body suffered some serious damage; damage that Yoda did not suffer.
While the darkside may be quicker and easier, it also seems to carry a terrible price for the wielder from a physical standpoint.