Consumption implies wastefulness, which we need to minimize. There are numerous vectors of value dynamics in between black-white saving-consumption.

Ideally, in a demurrage system "spending" from the spender's viewpoint would mean investment into more stable assets, like nybble41 said. Not consumption.

There's also a significant difference between debt and idle money [from work already done]. All debt systems are like playing with fire, someone will fail future production eventually even in the perfect setup of all good intentions. Maybe we should just focus on exchanging value already produced. And as importantly minimize idle value, subject to corrosion.