When Sherman turned Johnston out of the ridge country in Northwest Ga. and broke into the Etowah and Chattahoochee Valleys, and when Hood later left Palmetto Station, permitting Sherman the opportunity to finally turn the Appalachians and break into the AoNV's rear and threaten its right flank via an overland route from the southwest, sever the remaining communications between the corn cribs of the lower Chattahoochee and the Richmond depot, and threaten an overwhelming concentration on the Atlantic coast and compel the evacuation of the cities on it (which Lee had forecasted as early as Dec. 1863).
Although it is understandable why the Confederate leadership banked on a Tennessee expedition, the outcome proved so terrible and strategically suboptimal that the AoT proved itself simply unable to counter the grand raids of Sherman, Stoneman, and Wilson all at once in Feb-Apr. 1865, rapidly accelerating the collapse of the Confederacy beyond anyone's expectations in only a few weeks.
And as
@Wizard of Cozz said: "[Lee] only gets thinned out because he had to send troops to NC in early 65' because there was no AoT anymore. He sent Hoke's division, Kershaw's SC Brigade as well as two cavalry brigades. Nearly 10,000 troops. All because the west couldn't hold while he held the entire time..."