People planning this stuff aren't idiots and they knew that there was no scenario that Ukraine could defeat Russia militarily no matter what weapons they provided. For example, back in 2016, Obama declared Ukraine to be not a core American interest and that he is reluctant to intervene in the country, because Russia will always be able to maintain escalatory dominance there.

I don't think their goal was to create a drawn out conflict however. The idea was to use Ukraine as fodder and then break Russia economically. Western planners fundamentally misunderstood the nature of Russian economy, and thought that it would collapse after they froze Russian foreign assets and put sanctions on. Not only did that not happen, but major countries like China and India snubbed the west and continued trade with Russia. That's where the whole hare brained scheme started to come apart, but the time this became obvious the west was already too invested to pull back.