The problem is, to pull that off, Ukraine would need at least 2 times the Russian force, Russia has around 20k troop in the area, Ukraine is very tight lip about their formation, but general estimate usually put Ukrainian force at 5 to 6 Brigade, which is roughly 30,000 men. That is not enough to take on the defender. Unless Russia willingly retreats, this counter offensive is going to take a long time to complete. But all in all, unless Russia somehow manages to fix one of the bridges, I don't see how Russia can hold on indefinitely, they may be able to hold until winter came, but once fighting season start again next year, it will still be the same, their position is going to get pounded by HIMARS. And they would have problem bringing in reinforcement.