A very bad idea that would make invading Russia without consequences.
There's a military logic to it, of course.
1) Withdrawing along a predetermined corridor that we had left for them - the Ukrainians typically get turned into mincemeat by Russian artillery and FPV drones.
2) An encircled force will fight fiercely (remember Azovstal, or the Germans at Stalingrad). It will attempt multiple breakouts. Friendly forces on the outside will attempt to counterattack, and lift the siege. Russian forces that effect the encirclement, have the unenviable job of keeping the Ukrainians inside bottled up, while defending from the Ukrainians on the outside trying to relieve them.
3) It allows whatever is inside the encirclement to be taken more or less intact. If, instead, a full battle of annihilation is fought - the battle zone will turn into a lunar landscape. A few cities were taken intact recently, using these tactics of squeezing Ukrainians out by encircling them and leaving them only one exit route.
4) Whatever exit routes we leave the Ukrainians, are typically towards directions without cover or defensive lines. In other words, open fields.
Basically, not trying to capture or kill every single Ukrainian caught in a trap, may allow us to move much faster than otherwise.