Looking at what the US has sent to Ukraine, though, nearly all of it has been second tier equipment that was nearing retirement/replacement due to either being

Looking at what the US has sent to Ukraine, though, nearly all of it has been second tier equipment that was nearing retirement/replacement due to either being close to time expiring or because it was nearing obsolescence. The same is true for other countries. Even the F-16s being mentioned at the moment will be recently retired ones, not aircraft drawn from front line fleets. Javelins are one of the only front line systems, and not that much of a profit maker for the military industrial complex given the relatively low per unit price and according to reports it was the oldest in the inventory that were sent. The Raytheons and Boeings of the world would much rather the US be buying big ticket items like ships, fighter aircraft, and AMRAAMs than lower margin items like Javelins and 155mm artillery ammunition. Military systems go through a constant cycle of acquisition, usage, upgrades, retirement, and replacement. Some systems, like big aircraft with relatively undemanding flight and usage profiles, can be kept operational and upgraded for a long time. Others, like missiles, have shorter lifetimes and are less worth refurbishing and upgrading after a point. A certain proportion of what the US has donated was going to have to be retired, disposed of, and replaced in the next few years anyway.