Why do they need them? They were winning every day since the beginning.
What makes you think it will end before that? The russians want nato roll back from eastern europe. The war doesn't stop until that happens. Biden would rather die than give putin that. Given his age, it's actually possible.
To put it in perspective, when the invasion started, and for many months, Ukraine was never assumed to be "winning". Most experts and most people expected the war to be over in weeks.
What transpired shocked most people, particularly some respected experts. The Russian advance faltered in quite an extreme fashion, in the face of what was believed to be a much smaller and inferior enemy.
The following weeks and months saw Russia's most prestigious units and equipment decimated (
this was the real point where any rapid victory for Russia became impossible due to the timeframe of replacing these units).
Ukraine conducted several highly effective and successful counterattacks that forced the Russians to retreat wholly, or broke Russian forces to the point of complete rout (
this was the point people started to believe Ukraine could simply sweep through Russian forces at will).
Russia and both Ukraine become committed to the town of Bakhmut for various reasons. It appeared that Russia believed it would be a straightforward PR victory, but the cost began to increase dramatically. Ukraine was not initially interested, for months, until it was obvious Russia was willing to throw troops in. (
It is at this stage where Ukranian casualties begin to rise).
The winter months allowed Russia to build large defensive lines, and Ukraine waiting to receive equipment in order to build many new brigades gave them even longer (
this guaranteed that the Ukranian offensive would never be quick in front of the heavily mined 1st defensive position).
It's important to view the war in a much larger scale and over a longer time period. Ramifications will be long lasting (decades). Ukraine will likely benefit heavily from Western integration and investment. Russia may find new markets to rebuild it's economy, and might find new willing partners not attached to the US in the west, and China in the east. However, nothing is certain and we will probably be dealing with a situation we're not expecting. This will apply to Russia, Ukraine and the rest of the world.
As for NATO pulling back from Eastern Europe, this will not happen in any of our lifetimes. A new country (Finland) will create another NATO/Russia border. Whilst Ukraine is still a question mark in NATO, expect the Baltic States, Poland and probably some Western European powers to offer individual security guarantees as soon as the opportunity arises.
Putin will be long gone before NATO recedes.
Personally, I believe the full-scale invasion of Ukraine was a collosal blunder. In week 2-3 when it became apparent Russia lacked the organisation/doctrine to achieve a rapid victory, they should have withdrew back to the position they are currently holding, saving men and material, in order to use their position as a bargaining chip for whatever it was they wanted. I believe they didn't do this because they wanted Ukraine, nothing else.
This is in contrast to the capture of Crimea, which was an absolute masterclass in the (apparent) rapid capture of territory. The military forces involved where tiny compared to something like South Ossetia. This was what people were terrified of, Russia's version of hybrid warfare. It was quick, precise and virtually no fighting. I have even used it as an example in presentations because the relative force numbers are staggering. (Watch Vice: Russian Roulette. You can watch almost hour by hour of how obscenely difficult it was to counter. You can also see the difference between Ukranian military in 2014 vs 2022).
Long war? It's possible. We tend to focus too much on the battlefield. I believe this war will end in whatever state it is at, when things start to happen in Moscow and other capitals.
On the battlefield today Russia has held strongly over a very long line against a huge number of Ukrainian forces. They now have to work out what and how they are going to deal with the current Ukranian advanced. Things will begin to move quicker as this lines are penetrated, but Russia will have a plan on where to move and how to react to these breaches. It is not something they can stop, but they will have a plan.