A lot of the reporting on Ukraine speaks of “Russians withdrawing from X”. This choice of words understates the severity of the situation for the Russians.
A lot of these “withdrawals” were more akin to “routs” - units racing in a semi-organized or completely disorganized fashion to leave an encircled area, under artillery fire, leaving behind much of their equipment and virtually ALL of their ammunition and fuel. Of the units that formed the old front line, most will now have greatly diminished or no combat value as they are low on ammunition, fuel and equipment. These units cannot just fall back to form a new front. For the moment, they’re actually worthless until they can be resupplied.
The fact that the Russian logistics concept has been thoroughly disrupted by the Ukrainian offensive doesn’t help with that.
In a “normal” war, the defending side (Russia, in this case) would have some kind of defense in depth that can be beefed up to contain a breakthrough, and reserves to deploy to man those defenses. But Russia doesn’t seem to have a ready reserve it can deploy. Much of its reserve is trapped in Kherson, and if they start withdrawing troops from other areas, Ukraine will likely use its MUCH shorter internal lines to launch another offensive at the weakened points.