Most of the weapons are paid for and in inventory. Quite a few types were designed and manufactured primarily for a large war in Europe (as they are not what we would likely need in big numbers in a dustup with China, for example.) So if our weapons are destroying Russian weapons now, the idea is for Russia to be seriously weakened to the point of being unable to rebuild it’s own huge inventories, and then we won’t need those weapons of ours in, say, 2028, whether it be for use or deterrence vs. Russia. A minor point is that it’s probably cheaper to send the weapons to Ukraine than the costs of eventual removal from service or inventory.
Side note: Your info. on the Russian-Finnish conflicts is seriously incomplete. For starters:
https://en.wikipedia.org/wiki/Winter_War
https://en.wikipedia.org/wiki/Continuation_War
And go from there...
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The economic support to Ukraine is another matter and I believe that at most it should come in the form of long term payback loans to the EU or Euro countries we judge should be in a position to repay in 5-10 years: Those Euro countries can then decide in what way to loan or give the money to Ukraine. (Personally, I’d go for mineral rights concessions.)
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As for why back Ukraine? Like the present global (Western plus some allies like Japan, Korea, etc.) security order or not, without it, our problems are going to be a hell of a lot bigger if it breaks up. For starters, major nuclear proliferation amongst MANY countries is a given if Russia is not beaten back. If you don’t understand that, you just don’t understand how this world works, and doesn’t work. A thorough restudy of history might help.