well, by committing the reserves they would no doubt defeat the remaining legions, but at that point their manpower is just insufficient to maintain control over the entire continent, especially since you would have rather large and effective resistance movements rising to combat the vile elven supremicists.
The argument can be made that Medes II should have considered this, but for him to do that he would have had to have a rough idea of the type of reserves they had, and for that he'd need spies and intelligence networks. And for those, he'd need the Blades, who, due to their orders, but also likely do to arrogance, were all but wiped out for believing themselves more than a match for Thalmor agents.
So, to sum up, by the time Cyrodill is reclaimed:
-Medes II's legions are decimated
-the entire empire is war weary
-the cities of the empire lie in ruins
-likely thousands of her civillians have been murdered
-he has no intelligence on the Aldmeri Dominion's lands
-no idea the type of reserves they possess, but every idea the reserves the empire possesses: none.
the deck is just stacked against the empire too much. calling a possible bluff from the Thalmor would have possessed far too great of a risk to the survival and safety of all of tamriel.

just my humble opinion