What are you basing this statement on? All analysis I have seen so far come to the conclusion, that the war is devastating on an humanitarian and demographic scale, but sustainable for years to come from an pure manpower & training-capacity pov. (Given historical data)
The same analysts suggest that the main reason for the current stalemate is the lack of weapon systems/munitions, that would enable a sustained breakthrough like artillery, tanks and air defense systems. Stuff that western allies hesitate or are incapable to provide in large quantities.