Why would Russia tend to win a long war of Attrition?

For the same reason the Taliban won in Afghanistan. It became too expensive for the U.S. to continue that war any longer so we lost like we did in Viet Nam once again.

The same to some degree is true of the Ukraine war because Putin (as long as he is alive and ruling Russia) will never give up even if every man, woman and child in Russia is killed in that war. So, Putin's personal resolve is greater than the democracies resolve UNLESS a way is found to end this war soon. So, I think the western Democracies, NATO and the U.S. if they want Ukraine to be a part of NATO and the EU need to think about how to end this war and quickly before through attrition and being too expensive to continue ends this war in Russia's Favor by a long term (5 to 10 years war) of attrition.

Attrition is because Russia is right next to Ukraine and can supply weapons to it's troops more easily and the U.S. and Europe and Japan are too far away to do this economically no matter how efficient they are at doing this. So, the war eventually becomes too expensive to continue and the west de facto gives up Ukraine (even though they might not want to) in a similar way that we gave up Viet Nam and then Afghanistan.