Seems balanced to me... if they aren't warding the forest, killing your creeps, or taking any other precautions for such an extensive period of time... behold the army of the Legion's Queen!
Its no different than any other character who becomes insanely powerful if allowed to creep for extensive periods of time, its just manifested in an army of Bulls!
I'm sure a tank with really high survivability could still live through this with the right items, taking the same if not less amount of time to acquire, while also not being "drop on death." (considering once pets die, they are done.)