Thinkable.
The Russians have lost a lot of both men and equipment, and professional cadre besides.
Then there is the upcoming mud and winter weather, losing supply lines, evident lack of equipment and, surprisingly, ammo, etc.
They can expect a lot of infantry replacements, but that should help only defensively until they can make up their materiel deficiencies. Indeed the Russians may now be facing the same situation the Ukrainians were in from April-July - lots of men but little equipment and firepower.
If the Russians had had these infantry replacements six months ago it may have been a different story.
So the Russians arent about to defeat Ukraine in the next six months. At best they may be able to hold their ground. That will throw the question outside the scope of the battlefield and into the field of politics and economics.