Well that equipment will be replaced though. Just as Russia is digging out out their T-55s from storage the US military likes to have the capability to do something similar if things get desperate. Just with stuff that's 30 years not 60 years old like Russia is doing.
But they'd need to replace their current equipment with new stuff, they're current equipment will be put into storage.
And yeah they'd want to do this anyway as they don't want to get into a situation like Russia where their stored equipment is 60 years old. So you're kinda right, but if they're replacing equipment a little sooner than planned then there's a calculation for the cost that involves depreciation formulas and the like. Note this was the cause of that accounting error they had recently which was an insane amount of money. They didn't misplace money or misplace equipment, they simply miscalculated the depreciation of the old equipment.
So there is a cost despite it being old equipment. It's not strictly correct to say it's free, though it would be even more incorrect to say it's the same cost as new equipment. The cost is somewhere in between, and it's complicated enough to place a value on it that the DoD screwed it up.