making less durable goods better investments than just holding on to your money.
Yes, what's wrong with less durable goods as long as they're based on sustainability of the whole (which they currently aren't)?
Well, that's just the problem. Inflation and demurrage both encourage buying cheaper, disposable items, rather than saving up for one that will last longer. They encourage needless consumption.
Sure, but how do you propose I buy a car with productive activity?
You're right in that practically no value can be exchanged instantaneously. A medium of exchange, and necessarily storage of value is required. However, usually as soon as you store the value, it starts to erode (not talking only about money, but anything, because of entropy).
Then why do some things appreciate in value over time? Why is a bottle of wine that is 50 years old more valuable than one that is 5?
When you transfer value, its erosion is mitigated and - because of the dynamics and novelty (another basic property of life) that the value encounters instead of stagnation - it may be implicitly replenished (causing lessening of entropy within the system).
When applying this to your example of buying a car, maybe the excess value you produce daily / monthly that you intend to use to buy the car, should go to the car dealer immediately. Forward part-payments. You can be compensated in the total price for doing that. I don't know, just off my mind.
That is the car dealership acting as a bank, and managing your savings for you. Try again. How can I make a large purchase without saving or going into debt?