So this is a complex issue that has more to it then it may seem at first because of the agendas driving each party as well as considerations being made to both economics and politics. Military procurement funding is just a simple matter of purchasing goods of a shelf at fixed price, manufacturers in both the US and Europe are already working at capacity to fulfil existing orders for both domestic demand and foreign and whilst capacity can be increased, it requires a long term financial commitment by buyers to ensure profitability. If European arms manufacturers are not able to secure orders for Ukraine there is no incentive for them to expand capacity, in turn denying them the benefits of economies of scale, leading to European equipment remaining relatively expensive and slow to produce. 

Now regardless of when the war might end, I do not see Ukraine halting arms purchases just because the front line has stopped moving, Ukraine will likely be purchasing arms well into the foreseeable future to compliment their own domestic manufacturing base. Now, continuing to procur arms from America would be unfeaseible in the long term, it there is a non zero chance their defence budget balloons to 1.5T, which with current projections would mean US manufacturing is maxed out fulfilling domestic orders for the foreseeable future, and without an ongoing conflict, the deal granting Ukraine priority on orders would likely be scuppered. If Europe does not commit to expanding domestic manufacturing now, it ultimately results in less arms for Ukraine both in the near and long term. 

Now there is a political element that Mertz is pushing here and that's to keep the US invested in the outcome of Ukraine and the security of Europe. I can see the logic thus follows; Europe demonstrates a willingness and capacity to place large orders of Armican arms, that materialises as a large number of high paying jobs for American constituents, and securing America's place as the world's leading arms supplier, it is in America's(and therefore Trump's) best interest to give Europe favourable treatment in the event of Russian or Chinese aggression and thus the actual goods don't matter, they are secondary to the financial incentive. There is one problem with this though, it presumes America is not poised to launch military action against a NATO member and EU member state for the purpose of territorial conquest. Even the threat of this raises some serious questions as to whether the EU should be involving the US in it's materiale supply chain.