Why? These kind of cold-war era proxy wars happened throughout history. One big power makes a move directly or indirectly, while the other side will be supported indirectly by other powers. It becomes a pure resource war, and no party has a reason to escalate it beyond the conflict zone. Russia has no reason to open new fronts even if NATO props the war zone, or covertly supports sabotage attacks within Russia's border - the same way the US or China fights proxy wars, based on success, they take it or leave it (for the time). Ukraine is a long term strategic investment for Russia, not a life and death struggle.