Those are all absolutely valid points everyone is making (ok maybe not the likelihood of hordes of DRNK troops entering the fray).

I would simply suggest that even operational-level positional warfare is not necessarily a war of attrition. Warfare, particularly since the dawn of the industrial age, rarely is just about numbers, it is about combat power.
As someone noted, the Confederacy was indeed dramatically outnumbered, and once Lincoln found a general willing to take the casualties to achieve ultimate victory, Grant wore down the Army of Northern Virginia in the Overland Campaign of 1864/65. But was that really attritional warfare?
I would argue it was something very different. Though Grant inflicted unsustainable casualties on Lee and suffered enormous ones himself, he did it through operational maneuver. The few occasions he forgot that, such as Cold Harbor and the first attacks at Petersburg, it was he who faced unsustainable casualties and very nearly had a mutiny on his hands among his subordinates.
Lee showed absolute brilliance in the handling of his army following the battle of the Wilderness until the end of the war. Historians have often argued over what may have happened had Lee taken the lessons of Sharpsburg to heart in 1862 and gone over to the operational defensive for the remainder of the war. That would have looked far more like what we see in Ukraine today than anything that actually happened during that war. Without a Gettysburg, I am confident most of historians would agree that the war at least would not have ended in 1865. I think most historians would also agree that sustaining support for the war beyond 1865 would have been very difficult if not impossible.
History offers us numbers of other examples where the operational defensive likely would have been or was the correct strategy. For instance, in the war of the Sixth Coalition the army of France was outnumbered by those of the UK, Russia, and their allies. I can think of very few students of the Napoleonic campaigns who believe Napoleon would have died anywhere but his bed in Paris had he not taken the Grand Army on its Quixotic march into the heart of Russia.
To date at least, following the catastrophe of the initial invasion of Ukraine, the Russian Army has shown little ability to operationally maneuver against Ukrainian forces. Perhaps one could point to the battle of Bahkmut, but I would argue we have not yet seen the final outcome of that campaign. Indeed, I would suggest they have shown no ability to even mass combat power operationally to achieve a decisive battle.
I also wonder about the actual casualty exchange rates in this conflict. The figures we are citing are those being publicly discussed by the US and British DOD and MOD respectively. They are therefore conservative. But what if one side or the other is half again higher, perhaps even 2X? I find it highly unlikely that would be the case of Ukraine, but because they continue to attempt the tactical offensive, I would not at all be surprised if Russia isn't actually approaching, even exceeding half a million.
For instance, as of yesterday, Ukraine claims to have killed 306,860 Russian troops. I think everyone would agree that number is inflated. We'll come back to it in a moment. The next question is what is the Russian WIA to KIA rate? For US forces in the European Theater of Operations in WWII it was roughly 3 to 1. I suspect those rates are closer to the campaign in Ukraine than modern US ratios which can be 10 to 1. Russian field medical capability in particular seem far closer to mid-twentieth century than twenty first century standards.
So let's cut Ukraine's claim by in half to 150,000. Assuming that 3 to 1 casualty ratio, that means Russia has already suffered 600K total casualties in this conflict. Even if their field medical services are far worse than we believe, a 2 to 1 casualty rate leaves us with 450,000 total casualties.
Moreover, this is also a war of technology - of combat power. The vast majority of Russia's modernized army is scrap metal on the steppes of Ukraine. That means these columns of attacking forces that are being immolated almost daily on the Avdiivka front are going into battle with ever more ineffective and easily destroyed equipment. So excluding casualties for a moment, I don't think many informed observers would argue that the combat capability correlation of forces hasn't gradually been tipping in Ukraine's favor for most of the war. Therefore, in modern warfare, attrition is as much about erosion of combat power as numbers of troops - in my view, far more so.
So, we come back to casualties. Again, in my view, I think casualty rate has become the latest, or perhaps last refuge of those who have been saying Ukraine can't win this war. There are more Russians, Putin will never quit, ergo Russia wins.
We will see. I believe Putin has lost the window to mobilize additional forces beyond the usual annual draft. If politically, that window ever existed. PRNK troops are not going to be deployed in their tens of thousands to Ukraine. The troops he does have are fighting with ever more inferior equipment than that of their adversaries. That is a formula that looks far more like 1917 than 1945 to me.
One of these two combatants will begin to lobby for a ceasefire and negotiations. Thus far, it clearly is not Ukraine.