All the Russians have to do is dig trenches around surrounded cities and wait.
Americans generally lack patience.
Russian patience exceeds Ukrainian city food supplies.
“All the Russians have to do is dig trenches around surrounded cities and wait.”
Time is not on Russia’s side.
It is costing them dearly every day ($20 billion/day has been estimated), while they are losing money from sanctions. They have already accumulated more combat losses than the USA incurred in 20 years in Afghanistan. There are more anti-tank missiles in Ukrainian hands, than there are armored vehicles in the forces Russia has deployed, and more are on the way. The further they go into the Ukraine, the longer and more vulnerable their supply lines become.
The forces they surround the cities with, are themselves surrounded by a much larger Ukrainian population, any one of which could kill their vehicles with a Javelin (or one of the many equivalents now flowing into Ukraine). Without their armored vehicles, they are greatly outnumbered men on foot in a foreign country, looking for a meal and a safe place to sleep.