Well considering the limited venues of retreat, allowing them an exit might be wise to get a retreat/rout going.Conventionally you will need at least 3 to 1, but most of the time you can make do with 2 to 1, but then it would have been a bloody fight.
The motivation wouldn't really matter for a professional army, so the question will be whether or not Russia is professional enough. It seems like it's what Ukraine is betting on tho, that's Russian army is not really that professional, because as I said, from what I can see on the position in battlefield, seems like the Ukrainian is trying to encircle them and then either hitting their supply line and starve their will to fight by hitting their GLOC or level the city with the Russian in it. Either way, I don't sense that the Ukrainian is looking for a fight to finish off the Russian, seems like they want to encircle it, make the Russian know there are no escape, and they would have to pull out instead of perishing inside the city.
This route can then be saturated by Himars once they get piled up there?