Over the ~10 years of US involvement in Vietnam, the US had 58k dead and 150k hospitalized. In less than two and a half years of SMO, Russia has had probably around 315k casualties? 61k have been confirmed dead for sure.
The other thing to consider is that sure Russia had 2.1M professional soldiers and reservists, but how many knew how to fly a Ka-50? How many were well trained tank gunners? How many were competent, experienced members of the NCO corps, or well trained and equipped paratroopers, or artillery officers? How many are left, and how many have to have their positions go empty or be filled by someone new without adequate experience?
The other thing to consider is that sure Russia had 2.1M professional soldiers and reservists, but how many knew how to fly a Ka-50? How many were well trained tank gunners? How many were competent, experienced members of the NCO corps, or well trained and equipped paratroopers, or artillery officers? How many are left, and how many have to have their positions go empty or be filled by someone new without adequate experience?