A new theory I thought of today:
Mugetsu isn't the final getsuga tensho because it will cause the user to lose power, but it is final because after using it, the Ichigo learn a new power instead. A more awesome Ban Kai/ SS4. Isshin coincidently discovered how to do so without actually knowing it. The period after Mugetsu and SS4 the new power Ichigo experiences a loss of reiatsu because he can't recognize the new form his reiatsu has taken and so he can't use it for the time being.