Deflation doesn't mean there's no incentive to spend. It means there's a slight relative reduction in the incentive to spend. You can argue that it's still bad, but it's a very different thing.
If the value of the reserve money is $2b one year and it deflates to $3b the next, does that mean you can print more fiat every year? Sounds like an attractive proposition to me
If you got the money with zero energy expenditure, you don't care what the current fiat market value is. It's like finding a free lunch sack of cash on a park bench.
You could argue that there's an opportunity cost to letting the money stay parked, but because governments ultimately dictate the value of their own currency, that point is moot.