That will slow the Ukrainian advance again, but it probably won’t stop it because their foot-soldiers can still move across country. They won’t reach the Black Sea coast this year and physically cut off all the Russian occupation forces west of there, as they once hoped — but they may get far enough to starve the Russians.
Ukraine’s HIMARS rocket artillery can already reach the only railway line connecting Russia with its forces in the western parts of occupied Ukraine. If they advance just another 15 or 20kms, they will also be able to hit the main road west along the Black Sea coast with HIMARS.
Once Washington delivers the promised ATACMS long-range (300km) missiles, Ukraine will also be able to take out the Kerch Bridge from Russia to occupied Crimea. (The western half of the bridge is on Ukrainian territory, so Kyiv would not be breaking its promise to President Biden not to strike targets in Russia with US weapons.)
At that point, perhaps before the end of this year, around half of the Russian troops in Ukraine would be more or less cut off, on half-rations for supplies and ammunition at best. This is by no means certain, but it is a realistic possibility.
Russian generals will be aware of this danger, but although Russia has more than three times Ukraine’s population, its troops in Ukraine barely outnumber the Ukrainian defenders and they are close to exhaustion.
Winning this war is not vital for Russia. It’s just a foreign military adventure gone wrong, like Britain and France invading Egypt in 1956 or the US invading Afghanistan and Iraq in 2001-2003.
However, winning this war is vital for Putin’s personal position and perhaps even for his life, so there will be a point of maximum danger if and when he finds out he is going to lose. If he should decide to gamble everything on a potentially catastrophic nuclear escalation, would the people around him go along with it?
There is a lot of apocalyptic talk among the Russian elite right now, but this is the original homeland of “doublethink” (in George Orwell’s 1984). Orwell never visited Russia, but he got it exactly right: all those Russian talking heads who are now saying the survival of Russia is at stake also know that it’s not really threatened at all.
Which means they are unlikely to risk their families’ lives just for Putin’s survival.